Curated collection of useful JavaScript snippets that you can understand in 30 seconds or less.
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// CommonJS
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import _30s from '30-seconds-of-code';
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allallEqualanyarrayToCSVbifurcatebifurcateBychunkcompactcountBycountOccurrencesdeepFlattendifferencedifferenceBydifferenceWithdropdropRightdropRightWhiledropWhileeveryNthfilterFalsyfilterNonUniquefilterNonUniqueByfindLastfindLastIndexflattenforEachRightgroupByheadincludesAllincludesAnyindexOfAllinitialinitialize2DArrayinitializeArrayWithRangeinitializeArrayWithRangeRightinitializeArrayWithValuesinitializeNDArrayintersectionintersectionByintersectionWithisSortedjoinJSONtoCSVlastlongestItemmapObjectmaxNminNnonenthElementoffsetpartitionpermutationspullpullAtIndexpullAtValuepullByreducedFilterreduceSuccessivereduceWhichrejectremovesamplesampleSizeshankshufflesimilaritysortedIndexsortedIndexBysortedLastIndexsortedLastIndexBystableSortsymmetricDifferencesymmetricDifferenceBysymmetricDifferenceWithtailtaketakeRighttakeRightWhiletakeWhiletoHashunionunionByunionWithuniqueElementsuniqueElementsByuniqueElementsByRightuniqueSymmetricDifferenceunzipunzipWithwithoutxProdzipzipObjectzipWith
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arrayToHtmlListbottomVisiblecopyToClipboardcountercreateElementcreateEventHubcurrentURLdetectDeviceTypeelementContainselementIsVisibleInViewportformToObjectgetImagesgetScrollPositiongetStylehasClasshashBrowserhidehttpsRedirectinsertAfterinsertBeforeisBrowserTabFocusednodeListToArrayobserveMutationsoffononUserInputChangeprefixrecordAnimationFramesredirectrunAsyncscrollToTopserializeFormsetStyleshowsmoothScrolltoggleClasstriggerEventUUIDGeneratorBrowser
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approximatelyEqualaverageaverageBybinomialCoefficientclampNumberdegreesToRadsdigitizedistanceelofactorialfibonaccigcdgeometricProgressionhammingDistanceinRangeisDivisibleisEvenisNegativeZeroisOddisPrimelcmluhnCheckmapNumRangemaxBymedianmidpointminBypercentilepowersetprimesradsToDegreesrandomIntArrayInRangerandomIntegerInRangerandomNumberInRangeroundsdbmstandardDeviationsumsumBysumPowertoSafeIntegervectorDistance
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byteSizecapitalizecapitalizeEveryWordcompactWhitespaceCSVToArrayCSVToJSONdecapitalizeescapeHTMLescapeRegExpfromCamelCaseindentStringisAbsoluteURLisAnagramisLowerCaseisUpperCasemapStringmaskpadpalindromepluralizeremoveNonASCIIreverseStringsortCharactersInStringsplitLinesstringPermutationsstripHTMLTagstoCamelCasetoKebabCasetoSnakeCasetoTitleCasetruncateStringunescapeHTMLURLJoinwords
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Creates a function that accepts up to n arguments, ignoring any additional arguments.
Call the provided function, fn, with up to n arguments, using Array.prototype.slice(0, n) and the spread operator (...).
const ary = (fn, n) => (...args) => fn(...args.slice(0, n));Examples
const firstTwoMax = ary(Math.max, 2);
[[2, 6, 'a'], [6, 4, 8], [10]].map(x => firstTwoMax(...x)); // [6, 6, 10]Given a key and a set of arguments, call them when given a context. Primarily useful in composition.
Use a closure to call a stored key with stored arguments.
const call = (key, ...args) => context => context[key](...args);Examples
Promise.resolve([1, 2, 3])
.then(call('map', x => 2 * x))
.then(console.log); // [ 2, 4, 6 ]
const map = call.bind(null, 'map');
Promise.resolve([1, 2, 3])
.then(map(x => 2 * x))
.then(console.log); // [ 2, 4, 6 ]Changes a function that accepts an array into a variadic function.
Given a function, return a closure that collects all inputs into an array-accepting function.
const collectInto = fn => (...args) => fn(args);Examples
const Pall = collectInto(Promise.all.bind(Promise));
let p1 = Promise.resolve(1);
let p2 = Promise.resolve(2);
let p3 = new Promise(resolve => setTimeout(resolve, 2000, 3));
Pall(p1, p2, p3).then(console.log); // [1, 2, 3] (after about 2 seconds)Flip takes a function as an argument, then makes the first argument the last.
Return a closure that takes variadic inputs, and splices the last argument to make it the first argument before applying the rest.
const flip = fn => (first, ...rest) => fn(...rest, first);Examples
let a = { name: 'John Smith' };
let b = {};
const mergeFrom = flip(Object.assign);
let mergePerson = mergeFrom.bind(null, a);
mergePerson(b); // == b
b = {};
Object.assign(b, a); // == bCreates a function that invokes each provided function with the arguments it receives and returns the results.
Use Array.prototype.map() and Function.prototype.apply() to apply each function to the given arguments.
const over = (...fns) => (...args) => fns.map(fn => fn.apply(null, args));Examples
const minMax = over(Math.min, Math.max);
minMax(1, 2, 3, 4, 5); // [1,5]Creates a function that invokes the provided function with its arguments transformed.
Use Array.prototype.map() to apply transforms to args in combination with the spread operator (...) to pass the transformed arguments to fn.
const overArgs = (fn, transforms) => (...args) => fn(...args.map((val, i) => transforms[i](val)));Examples
const square = n => n * n;
const double = n => n * 2;
const fn = overArgs((x, y) => [x, y], [square, double]);
fn(9, 3); // [81, 6]Performs left-to-right function composition for asynchronous functions.
Use Array.prototype.reduce() with the spread operator (...) to perform left-to-right function composition using Promise.then().
The functions can return a combination of: simple values, Promise's, or they can be defined as async ones returning through await.
All functions must be unary.
const pipeAsyncFunctions = (...fns) => arg => fns.reduce((p, f) => p.then(f), Promise.resolve(arg));Examples
const sum = pipeAsyncFunctions(
x => x + 1,
x => new Promise(resolve => setTimeout(() => resolve(x + 2), 1000)),
x => x + 3,
async x => (await x) + 4
);
(async() => {
console.log(await sum(5)); // 15 (after one second)
})();Performs left-to-right function composition.
Use Array.prototype.reduce() with the spread operator (...) to perform left-to-right function composition.
The first (leftmost) function can accept one or more arguments; the remaining functions must be unary.
const pipeFunctions = (...fns) => fns.reduce((f, g) => (...args) => g(f(...args)));Examples
const add5 = x => x + 5;
const multiply = (x, y) => x * y;
const multiplyAndAdd5 = pipeFunctions(multiply, add5);
multiplyAndAdd5(5, 2); // 15Converts an asynchronous function to return a promise.
In Node 8+, you can use util.promisify
Use currying to return a function returning a Promise that calls the original function.
Use the ...rest operator to pass in all the parameters.
const promisify = func => (...args) =>
new Promise((resolve, reject) =>
func(...args, (err, result) => (err ? reject(err) : resolve(result)))
);Examples
const delay = promisify((d, cb) => setTimeout(cb, d));
delay(2000).then(() => console.log('Hi!')); // // Promise resolves after 2sCreates a function that invokes the provided function with its arguments arranged according to the specified indexes.
Use Array.prototype.map() to reorder arguments based on indexes in combination with the spread operator (...) to pass the transformed arguments to fn.
const rearg = (fn, indexes) => (...args) => fn(...indexes.map(i => args[i]));Examples
var rearged = rearg(
function(a, b, c) {
return [a, b, c];
},
[2, 0, 1]
);
rearged('b', 'c', 'a'); // ['a', 'b', 'c']Takes a variadic function and returns a closure that accepts an array of arguments to map to the inputs of the function.
Use closures and the spread operator (...) to map the array of arguments to the inputs of the function.
const spreadOver = fn => argsArr => fn(...argsArr);Examples
const arrayMax = spreadOver(Math.max);
arrayMax([1, 2, 3]); // 3Creates a function that accepts up to one argument, ignoring any additional arguments.
Call the provided function, fn, with just the first argument given.
const unary = fn => val => fn(val);Examples
['6', '8', '10'].map(unary(parseInt)); // [6, 8, 10]Returns true if the provided predicate function returns true for all elements in a collection, false otherwise.
Use Array.prototype.every() to test if all elements in the collection return true based on fn.
Omit the second argument, fn, to use Boolean as a default.
const all = (arr, fn = Boolean) => arr.every(fn);Examples
all([4, 2, 3], x => x > 1); // true
all([1, 2, 3]); // trueCheck if all elements in an array are equal.
Use Array.prototype.every() to check if all the elements of the array are the same as the first one.
Elements in the array are compared using the strict comparison operator, which does not account for NaN self-inequality.
const allEqual = arr => arr.every(val => val === arr[0]);Examples
allEqual([1, 2, 3, 4, 5, 6]); // false
allEqual([1, 1, 1, 1]); // trueReturns true if the provided predicate function returns true for at least one element in a collection, false otherwise.
Use Array.prototype.some() to test if any elements in the collection return true based on fn.
Omit the second argument, fn, to use Boolean as a default.
const any = (arr, fn = Boolean) => arr.some(fn);Examples
any([0, 1, 2, 0], x => x >= 2); // true
any([0, 0, 1, 0]); // trueConverts a 2D array to a comma-separated values (CSV) string.
Use Array.prototype.map() and Array.prototype.join(delimiter) to combine individual 1D arrays (rows) into strings.
Use Array.prototype.join('\n') to combine all rows into a CSV string, separating each row with a newline.
Omit the second argument, delimiter, to use a default delimiter of ,.
const arrayToCSV = (arr, delimiter = ',') =>
arr
.map(v => v.map(x => (isNaN(x) ? `"${x.replace(/"/g, '""')}"` : x)).join(delimiter))
.join('\n');Examples
arrayToCSV([['a', 'b'], ['c', 'd']]); // '"a","b"\n"c","d"'
arrayToCSV([['a', 'b'], ['c', 'd']], ';'); // '"a";"b"\n"c";"d"'
arrayToCSV([['a', '"b" great'], ['c', 3.1415]]); // '"a","""b"" great"\n"c",3.1415'Splits values into two groups. If an element in filter is truthy, the corresponding element in the collection belongs to the first group; otherwise, it belongs to the second group.
Use Array.prototype.reduce() and Array.prototype.push() to add elements to groups, based on filter.
const bifurcate = (arr, filter) =>
arr.reduce((acc, val, i) => (acc[filter[i] ? 0 : 1].push(val), acc), [[], []]);Examples
bifurcate(['beep', 'boop', 'foo', 'bar'], [true, true, false, true]); // [ ['beep', 'boop', 'bar'], ['foo'] ]Splits values into two groups according to a predicate function, which specifies which group an element in the input collection belongs to. If the predicate function returns a truthy value, the collection element belongs to the first group; otherwise, it belongs to the second group.
Use Array.prototype.reduce() and Array.prototype.push() to add elements to groups, based on the value returned by fn for each element.
const bifurcateBy = (arr, fn) =>
arr.reduce((acc, val, i) => (acc[fn(val, i) ? 0 : 1].push(val), acc), [[], []]);Examples
bifurcateBy(['beep', 'boop', 'foo', 'bar'], x => x[0] === 'b'); // [ ['beep', 'boop', 'bar'], ['foo'] ]Chunks an array into smaller arrays of a specified size.
Use Array.from() to create a new array, that fits the number of chunks that will be produced.
Use Array.prototype.slice() to map each element of the new array to a chunk the length of size.
If the original array can't be split evenly, the final chunk will contain the remaining elements.
const chunk = (arr, size) =>
Array.from({ length: Math.ceil(arr.length / size) }, (v, i) =>
arr.slice(i * size, i * size + size)
);Examples
chunk([1, 2, 3, 4, 5], 2); // [[1,2],[3,4],[5]]Removes falsy values from an array.
Use Array.prototype.filter() to filter out falsy values (false, null, 0, "", undefined, and NaN).
const compact = arr => arr.filter(Boolean);Examples
compact([0, 1, false, 2, '', 3, 'a', 'e' * 23, NaN, 's', 34]); // [ 1, 2, 3, 'a', 's', 34 ]Groups the elements of an array based on the given function and returns the count of elements in each group.
Use Array.prototype.map() to map the values of an array to a function or property name.
Use Array.prototype.reduce() to create an object, where the keys are produced from the mapped results.
const countBy = (arr, fn) =>
arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => {
acc[val] = (acc[val] || 0) + 1;
return acc;
}, {});Examples
countBy([6.1, 4.2, 6.3], Math.floor); // {4: 1, 6: 2}
countBy(['one', 'two', 'three'], 'length'); // {3: 2, 5: 1}Counts the occurrences of a value in an array.
Use Array.prototype.reduce() to increment a counter each time you encounter the specific value inside the array.
const countOccurrences = (arr, val) => arr.reduce((a, v) => (v === val ? a + 1 : a), 0);Examples
countOccurrences([1, 1, 2, 1, 2, 3], 1); // 3Deep flattens an array.
Use recursion.
Use Array.prototype.concat() with an empty array ([]) and the spread operator (...) to flatten an array.
Recursively flatten each element that is an array.
const deepFlatten = arr => [].concat(...arr.map(v => (Array.isArray(v) ? deepFlatten(v) : v)));Examples
deepFlatten([1, [2], [[3], 4], 5]); // [1,2,3,4,5]Returns the difference between two arrays.
Create a Set from b, then use Array.prototype.filter() on a to only keep values not contained in b.
const difference = (a, b) => {
const s = new Set(b);
return a.filter(x => !s.has(x));
};Examples
difference([1, 2, 3], [1, 2, 4]); // [3]Returns the difference between two arrays, after applying the provided function to each array element of both.
Create a Set by applying fn to each element in b, then use Array.prototype.map() to apply fn to each element in a, then Array.prototype.filter()
const differenceBy = (a, b, fn) => {
const s = new Set(b.map(fn));
return a.map(fn).filter(el => !s.has(el));
};Examples
differenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [1]
differenceBy([{ x: 2 }, { x: 1 }], [{ x: 1 }], v => v.x); // [2]Filters out all values from an array for which the comparator function does not return true.
Use Array.prototype.filter() and Array.prototype.findIndex() to find the appropriate values.
const differenceWith = (arr, val, comp) => arr.filter(a => val.findIndex(b => comp(a, b)) === -1);Examples
differenceWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0], (a, b) => Math.round(a) === Math.round(b)); // [1, 1.2]Returns a new array with n elements removed from the left.
Use Array.prototype.slice() to remove the specified number of elements from the left.
const drop = (arr, n = 1) => arr.slice(n);Examples
drop([1, 2, 3]); // [2,3]
drop([1, 2, 3], 2); // [3]
drop([1, 2, 3], 42); // []Returns a new array with n elements removed from the right.
Use Array.prototype.slice() to remove the specified number of elements from the right.
const dropRight = (arr, n = 1) => arr.slice(0, -n);Examples
dropRight([1, 2, 3]); // [1,2]
dropRight([1, 2, 3], 2); // [1]
dropRight([1, 2, 3], 42); // []Removes elements from the end of an array until the passed function returns true. Returns the remaining elements in the array.
Loop through the array, using Array.prototype.slice() to drop the last element of the array until the returned value from the function is true.
Returns the remaining elements.
const dropRightWhile = (arr, func) => {
let rightIndex = arr.length;
while (rightIndex-- && !func(arr[rightIndex]));
return arr.slice(0, rightIndex + 1);
};Examples
dropRightWhile([1, 2, 3, 4], n => n < 3); // [1, 2]Removes elements in an array until the passed function returns true. Returns the remaining elements in the array.
Loop through the array, using Array.prototype.slice() to drop the first element of the array until the returned value from the function is true.
Returns the remaining elements.
const dropWhile = (arr, func) => {
while (arr.length > 0 && !func(arr[0])) arr = arr.slice(1);
return arr;
};Examples
dropWhile([1, 2, 3, 4], n => n >= 3); // [3,4]Returns every nth element in an array.
Use Array.prototype.filter() to create a new array that contains every nth element of a given array.
const everyNth = (arr, nth) => arr.filter((e, i) => i % nth === nth - 1);Examples
everyNth([1, 2, 3, 4, 5, 6], 2); // [ 2, 4, 6 ]Filters out the falsy values in an array.
Use Array.prototype.filter() to get an array containing only truthy values.
const filterFalsy = arr => arr.filter(Boolean);Examples
filterFalsy(['', true, {}, false, 'sample', 1, 0]); // [true, {}, 'sample', 1]Filters out the non-unique values in an array.
Use Array.prototype.filter() for an array containing only the unique values.
const filterNonUnique = arr => arr.filter(i => arr.indexOf(i) === arr.lastIndexOf(i));Examples
filterNonUnique([1, 2, 2, 3, 4, 4, 5]); // [1, 3, 5]Filters out the non-unique values in an array, based on a provided comparator function.
Use Array.prototype.filter() and Array.prototype.every() for an array containing only the unique values, based on the comparator function, fn.
The comparator function takes four arguments: the values of the two elements being compared and their indexes.
const filterNonUniqueBy = (arr, fn) =>
arr.filter((v, i) => arr.every((x, j) => (i === j) === fn(v, x, i, j)));Examples
filterNonUniqueBy(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 1, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id == b.id
); // [ { id: 2, value: 'c' } ]Returns the last element for which the provided function returns a truthy value.
Use Array.prototype.filter() to remove elements for which fn returns falsy values, Array.prototype.pop() to get the last one.
const findLast = (arr, fn) => arr.filter(fn).pop();Examples
findLast([1, 2, 3, 4], n => n % 2 === 1); // 3Returns the index of the last element for which the provided function returns a truthy value.
Use Array.prototype.map() to map each element to an array with its index and value.
Use Array.prototype.filter() to remove elements for which fn returns falsy values, Array.prototype.pop() to get the last one.
-1 is the default value when not found.
const findLastIndex = (arr, fn) =>
(arr
.map((val, i) => [i, val])
.filter(([i, val]) => fn(val, i, arr))
.pop() || [-1])[0];Examples
findLastIndex([1, 2, 3, 4], n => n % 2 === 1); // 2 (index of the value 3)
findLastIndex([1, 2, 3, 4], n => n === 5); // -1 (default value when not found)Flattens an array up to the specified depth.
Use recursion, decrementing depth by 1 for each level of depth.
Use Array.prototype.reduce() and Array.prototype.concat() to merge elements or arrays.
Base case, for depth equal to 1 stops recursion.
Omit the second argument, depth to flatten only to a depth of 1 (single flatten).
const flatten = (arr, depth = 1) =>
arr.reduce((a, v) => a.concat(depth > 1 && Array.isArray(v) ? flatten(v, depth - 1) : v), []);Examples
flatten([1, [2], 3, 4]); // [1, 2, 3, 4]
flatten([1, [2, [3, [4, 5], 6], 7], 8], 2); // [1, 2, 3, [4, 5], 6, 7, 8]Executes a provided function once for each array element, starting from the array's last element.
Use Array.prototype.slice(0) to clone the given array, Array.prototype.reverse() to reverse it and Array.prototype.forEach() to iterate over the reversed array.
const forEachRight = (arr, callback) =>
arr
.slice(0)
.reverse()
.forEach(callback);Examples
forEachRight([1, 2, 3, 4], val => console.log(val)); // '4', '3', '2', '1'Groups the elements of an array based on the given function.
Use Array.prototype.map() to map the values of an array to a function or property name.
Use Array.prototype.reduce() to create an object, where the keys are produced from the mapped results.
const groupBy = (arr, fn) =>
arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
acc[val] = (acc[val] || []).concat(arr[i]);
return acc;
}, {});Examples
groupBy([6.1, 4.2, 6.3], Math.floor); // {4: [4.2], 6: [6.1, 6.3]}
groupBy(['one', 'two', 'three'], 'length'); // {3: ['one', 'two'], 5: ['three']}Returns the head of a list.
Use arr[0] to return the first element of the passed array.
const head = arr => arr[0];Examples
head([1, 2, 3]); // 1Returns true if all the elements ιν values are included in arr, false otherwise.
Use Array.prototype.every() and Array.prototype.includes() to check if all elements of values are included in arr.
const includesAll = (arr, values) => values.every(v => arr.includes(v));Examples
includesAll([1, 2, 3, 4], [1, 4]); // true
includesAll([1, 2, 3, 4], [1, 5]); // falseReturns true if at least one element of values is included in arr , false otherwise.
Use Array.prototype.some() and Array.prototype.includes() to check if at least one element of values is included in arr.
const includesAny = (arr, values) => values.some(v => arr.includes(v));Examples
includesAny([1, 2, 3, 4], [2, 9]); // true
includesAny([1, 2, 3, 4], [8, 9]); // falseReturns all indices of val in an array.
If val never occurs, returns [].
Use Array.prototype.reduce() to loop over elements and store indices for matching elements.
Return the array of indices.
const indexOfAll = (arr, val) => arr.reduce((acc, el, i) => (el === val ? [...acc, i] : acc), []);Examples
indexOfAll([1, 2, 3, 1, 2, 3], 1); // [0,3]
indexOfAll([1, 2, 3], 4); // []Returns all the elements of an array except the last one.
Use arr.slice(0,-1) to return all but the last element of the array.
const initial = arr => arr.slice(0, -1);Examples
initial([1, 2, 3]); // [1,2]Initializes a 2D array of given width and height and value.
Use Array.prototype.map() to generate h rows where each is a new array of size w initialize with value. If the value is not provided, default to null.
const initialize2DArray = (w, h, val = null) =>
Array.from({ length: h }).map(() => Array.from({ length: w }).fill(val));Examples
initialize2DArray(2, 2, 0); // [[0,0], [0,0]]Initializes an array containing the numbers in the specified range where start and end are inclusive with their common difference step.
Use Array.from() to create an array of the desired length, (end - start + 1)/step, and a map function to fill it with the desired values in the given range.
You can omit start to use a default value of 0.
You can omit step to use a default value of 1.
const initializeArrayWithRange = (end, start = 0, step = 1) =>
Array.from({ length: Math.ceil((end - start + 1) / step) }, (v, i) => i * step + start);Examples
initializeArrayWithRange(5); // [0,1,2,3,4,5]
initializeArrayWithRange(7, 3); // [3,4,5,6,7]
initializeArrayWithRange(9, 0, 2); // [0,2,4,6,8]Initializes an array containing the numbers in the specified range (in reverse) where start and end are inclusive with their common difference step.
Use Array.from(Math.ceil((end+1-start)/step)) to create an array of the desired length(the amounts of elements is equal to (end-start)/step or (end+1-start)/step for inclusive end), Array.prototype.map() to fill with the desired values in a range.
You can omit start to use a default value of 0.
You can omit step to use a default value of 1.
const initializeArrayWithRangeRight = (end, start = 0, step = 1) =>
Array.from({ length: Math.ceil((end + 1 - start) / step) }).map(
(v, i, arr) => (arr.length - i - 1) * step + start
);Examples
initializeArrayWithRangeRight(5); // [5,4,3,2,1,0]
initializeArrayWithRangeRight(7, 3); // [7,6,5,4,3]
initializeArrayWithRangeRight(9, 0, 2); // [8,6,4,2,0]Initializes and fills an array with the specified values.
Use Array(n) to create an array of the desired length, fill(v) to fill it with the desired values.
You can omit val to use a default value of 0.
const initializeArrayWithValues = (n, val = 0) => Array(n).fill(val);Examples
initializeArrayWithValues(5, 2); // [2, 2, 2, 2, 2]Create a n-dimensional array with given value.
Use recursion.
Use Array.prototype.map() to generate rows where each is a new array initialized using initializeNDArray.
const initializeNDArray = (val, ...args) =>
args.length === 0
? val
: Array.from({ length: args[0] }).map(() => initializeNDArray(val, ...args.slice(1)));Examples
initializeNDArray(1, 3); // [1,1,1]
initializeNDArray(5, 2, 2, 2); // [[[5,5],[5,5]],[[5,5],[5,5]]]Returns a list of elements that exist in both arrays.
Create a Set from b, then use Array.prototype.filter() on a to only keep values contained in b.
const intersection = (a, b) => {
const s = new Set(b);
return a.filter(x => s.has(x));
};Examples
intersection([1, 2, 3], [4, 3, 2]); // [2, 3]Returns a list of elements that exist in both arrays, after applying the provided function to each array element of both.
Create a Set by applying fn to all elements in b, then use Array.prototype.filter() on a to only keep elements, which produce values contained in b when fn is applied to them.
const intersectionBy = (a, b, fn) => {
const s = new Set(b.map(fn));
return a.filter(x => s.has(fn(x)));
};Examples
intersectionBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [2.1]Returns a list of elements that exist in both arrays, using a provided comparator function.
Use Array.prototype.filter() and Array.prototype.findIndex() in combination with the provided comparator to determine intersecting values.
const intersectionWith = (a, b, comp) => a.filter(x => b.findIndex(y => comp(x, y)) !== -1);Examples
intersectionWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0, 3.9], (a, b) => Math.round(a) === Math.round(b)); // [1.5, 3, 0]Returns 1 if the array is sorted in ascending order, -1 if it is sorted in descending order or 0 if it is not sorted.
Calculate the ordering direction for the first two elements.
Use Object.entries() to loop over array objects and compare them in pairs.
Return 0 if the direction changes or the direction if the last element is reached.
const isSorted = arr => {
let direction = -(arr[0] - arr[1]);
for (let [i, val] of arr.entries()) {
direction = !direction ? -(arr[i - 1] - arr[i]) : direction;
if (i === arr.length - 1) return !direction ? 0 : direction;
else if ((val - arr[i + 1]) * direction > 0) return 0;
}
};Examples
isSorted([0, 1, 2, 2]); // 1
isSorted([4, 3, 2]); // -1
isSorted([4, 3, 5]); // 0Joins all elements of an array into a string and returns this string. Uses a separator and an end separator.
Use Array.prototype.reduce() to combine elements into a string.
Omit the second argument, separator, to use a default separator of ','.
Omit the third argument, end, to use the same value as separator by default.
const join = (arr, separator = ',', end = separator) =>
arr.reduce(
(acc, val, i) =>
i === arr.length - 2
? acc + val + end
: i === arr.length - 1
? acc + val
: acc + val + separator,
''
);Examples
join(['pen', 'pineapple', 'apple', 'pen'], ',', '&'); // "pen,pineapple,apple&pen"
join(['pen', 'pineapple', 'apple', 'pen'], ','); // "pen,pineapple,apple,pen"
join(['pen', 'pineapple', 'apple', 'pen']); // "pen,pineapple,apple,pen"
Converts an array of objects to a comma-separated values (CSV) string that contains only the columns specified.
Use Array.prototype.join(delimiter) to combine all the names in columns to create the first row.
Use Array.prototype.map() and Array.prototype.reduce() to create a row for each object, substituting non-existent values with empty strings and only mapping values in columns.
Use Array.prototype.join('\n') to combine all rows into a string.
Omit the third argument, delimiter, to use a default delimiter of ,.
const JSONtoCSV = (arr, columns, delimiter = ',') =>
[
columns.join(delimiter),
...arr.map(obj =>
columns.reduce(
(acc, key) => `${acc}${!acc.length ? '' : delimiter}"${!obj[key] ? '' : obj[key]}"`,
''
)
)
].join('\n');Examples
JSONtoCSV([{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }], ['a', 'b']); // 'a,b\n"1","2"\n"3","4"\n"6",""\n"","7"'
JSONtoCSV([{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }], ['a', 'b'], ';'); // 'a;b\n"1";"2"\n"3";"4"\n"6";""\n"";"7"'Returns the last element in an array.
Use arr.length - 1 to compute the index of the last element of the given array and returning it.
const last = arr => arr[arr.length - 1];Examples
last([1, 2, 3]); // 3Takes any number of iterable objects or objects with a length property and returns the longest one.
If multiple objects have the same length, the first one will be returned.
Returns undefined if no arguments are provided.
Use Array.prototype.reduce(), comparing the length of objects to find the longest one.
const longestItem = (...vals) => vals.reduce((a, x) => (x.length > a.length ? x : a));Examples
longestItem('this', 'is', 'a', 'testcase'); // 'testcase'
longestItem(...['a', 'ab', 'abc']); // 'abc'
longestItem(...['a', 'ab', 'abc'], 'abcd'); // 'abcd'
longestItem([1, 2, 3], [1, 2], [1, 2, 3, 4, 5]); // [1, 2, 3, 4, 5]
longestItem([1, 2, 3], 'foobar'); // 'foobar'Maps the values of an array to an object using a function, where the key-value pairs consist of the stringified value as the key and the mapped value.
Use an anonymous inner function scope to declare an undefined memory space, using closures to store a return value. Use a new Array to store the array with a map of the function over its data set and a comma operator to return a second step, without needing to move from one context to another (due to closures and order of operations).
const mapObject = (arr, fn) =>
(a => (
(a = [arr, arr.map(fn)]), a[0].reduce((acc, val, ind) => ((acc[val] = a[1][ind]), acc), {})
))();Examples
const squareIt = arr => mapObject(arr, a => a * a);
squareIt([1, 2, 3]); // { 1: 1, 2: 4, 3: 9 }Returns the n maximum elements from the provided array.
If n is greater than or equal to the provided array's length, then return the original array (sorted in descending order).
Use Array.prototype.sort() combined with the spread operator (...) to create a shallow clone of the array and sort it in descending order.
Use Array.prototype.slice() to get the specified number of elements.
Omit the second argument, n, to get a one-element array.
const maxN = (arr, n = 1) => [...arr].sort((a, b) => b - a).slice(0, n);Examples
maxN([1, 2, 3]); // [3]
maxN([1, 2, 3], 2); // [3,2]Returns the n minimum elements from the provided array.
If n is greater than or equal to the provided array's length, then return the original array (sorted in ascending order).
Use Array.prototype.sort() combined with the spread operator (...) to create a shallow clone of the array and sort it in ascending order.
Use Array.prototype.slice() to get the specified number of elements.
Omit the second argument, n, to get a one-element array.
const minN = (arr, n = 1) => [...arr].sort((a, b) => a - b).slice(0, n);Examples
minN([1, 2, 3]); // [1]
minN([1, 2, 3], 2); // [1,2]Returns true if the provided predicate function returns false for all elements in a collection, false otherwise.
Use Array.prototype.some() to test if any elements in the collection return true based on fn.
Omit the second argument, fn, to use Boolean as a default.
const none = (arr, fn = Boolean) => !arr.some(fn);Examples
none([0, 1, 3, 0], x => x == 2); // true
none([0, 0, 0]); // trueReturns the nth element of an array.
Use Array.prototype.slice() to get an array containing the nth element at the first place.
If the index is out of bounds, return undefined.
Omit the second argument, n, to get the first element of the array.
const nthElement = (arr, n = 0) => (n === -1 ? arr.slice(n) : arr.slice(n, n + 1))[0];Examples
nthElement(['a', 'b', 'c'], 1); // 'b'
nthElement(['a', 'b', 'b'], -3); // 'a'Moves the specified amount of elements to the end of the array.
Use Array.prototype.slice() twice to get the elements after the specified index and the elements before that.
Use the spread operator(...) to combine the two into one array.
If offset is negative, the elements will be moved from end to start.
const offset = (arr, offset) => [...arr.slice(offset), ...arr.slice(0, offset)];Examples
offset([1, 2, 3, 4, 5], 2); // [3, 4, 5, 1, 2]
offset([1, 2, 3, 4, 5], -2); // [4, 5, 1, 2, 3]Groups the elements into two arrays, depending on the provided function's truthiness for each element.
Use Array.prototype.reduce() to create an array of two arrays.
Use Array.prototype.push() to add elements for which fn returns true to the first array and elements for which fn returns false to the second one.
const partition = (arr, fn) =>
arr.reduce(
(acc, val, i, arr) => {
acc[fn(val, i, arr) ? 0 : 1].push(val);
return acc;
},
[[], []]
);Examples
const users = [{ user: 'barney', age: 36, active: false }, { user: 'fred', age: 40, active: true }];
partition(users, o => o.active); // [[{ 'user': 'fred', 'age': 40, 'active': true }],[{ 'user': 'barney', 'age': 36, 'active': false }]]Generates all permutations of an array's elements (contains duplicates).
Use recursion.
For each element in the given array, create all the partial permutations for the rest of its elements.
Use Array.prototype.map() to combine the element with each partial permutation, then Array.prototype.reduce() to combine all permutations in one array.
Base cases are for array length equal to 2 or 1.
const permutations = arr => {
if (arr.length <= 2) return arr.length === 2 ? [arr, [arr[1], arr[0]]] : arr;
return arr.reduce(
(acc, item, i) =>
acc.concat(
permutations([...arr.slice(0, i), ...arr.slice(i + 1)]).map(val => [item, ...val])
),
[]
);
};Examples
permutations([1, 33, 5]); // [ [ 1, 33, 5 ], [ 1, 5, 33 ], [ 33, 1, 5 ], [ 33, 5, 1 ], [ 5, 1, 33 ], [ 5, 33, 1 ] ]Mutates the original array to filter out the values specified.
Use Array.prototype.filter() and Array.prototype.includes() to pull out the values that are not needed.
Use Array.prototype.length = 0 to mutate the passed in an array by resetting it's length to zero and Array.prototype.push() to re-populate it with only the pulled values.
const pull = (arr, ...args) => {
let argState = Array.isArray(args[0]) ? args[0] : args;
let pulled = arr.filter((v, i) => !argState.includes(v));
arr.length = 0;
pulled.forEach(v => arr.push(v));
};Examples
let myArray = ['a', 'b', 'c', 'a', 'b', 'c'];
pull(myArray, 'a', 'c'); // myArray = [ 'b', 'b' ]Mutates the original array to filter out the values at the specified indexes.
Use Array.prototype.filter() and Array.prototype.includes() to pull out the values that are not needed.
Use Array.prototype.length = 0 to mutate the passed in an array by resetting it's length to zero and Array.prototype.push() to re-populate it with only the pulled values.
Use Array.prototype.push() to keep track of pulled values
const pullAtIndex = (arr, pullArr) => {
let removed = [];
let pulled = arr
.map((v, i) => (pullArr.includes(i) ? removed.push(v) : v))
.filter((v, i) => !pullArr.includes(i));
arr.length = 0;
pulled.forEach(v => arr.push(v));
return removed;
};Examples
let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtIndex(myArray, [1, 3]); // myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]Mutates the original array to filter out the values specified. Returns the removed elements.
Use Array.prototype.filter() and Array.prototype.includes() to pull out the values that are not needed.
Use Array.prototype.length = 0 to mutate the passed in an array by resetting it's length to zero and Array.prototype.push() to re-populate it with only the pulled values.
Use Array.prototype.push() to keep track of pulled values
const pullAtValue = (arr, pullArr) => {
let removed = [],
pushToRemove = arr.forEach((v, i) => (pullArr.includes(v) ? removed.push(v) : v)),
mutateTo = arr.filter((v, i) => !pullArr.includes(v));
arr.length = 0;
mutateTo.forEach(v => arr.push(v));
return removed;
};Examples
let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtValue(myArray, ['b', 'd']); // myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]Mutates the original array to filter out the values specified, based on a given iterator function.
Check if the last argument provided in a function.
Use Array.prototype.map() to apply the iterator function fn to all array elements.
Use Array.prototype.filter() and Array.prototype.includes() to pull out the values that are not needed.
Use Array.prototype.length = 0 to mutate the passed in an array by resetting it's length to zero and Array.prototype.push() to re-populate it with only the pulled values.
const pullBy = (arr, ...args) => {
const length = args.length;
let fn = length > 1 ? args[length - 1] : undefined;
fn = typeof fn == 'function' ? (args.pop(), fn) : undefined;
let argState = (Array.isArray(args[0]) ? args[0] : args).map(val => fn(val));
let pulled = arr.filter((v, i) => !argState.includes(fn(v)));
arr.length = 0;
pulled.forEach(v => arr.push(v));
};Examples
var myArray = [{ x: 1 }, { x: 2 }, { x: 3 }, { x: 1 }];
pullBy(myArray, [{ x: 1 }, { x: 3 }], o => o.x); // myArray = [{ x: 2 }]Filter an array of objects based on a condition while also filtering out unspecified keys.
Use Array.prototype.filter() to filter the array based on the predicate fn so that it returns the objects for which the condition returned a truthy value.
On the filtered array, use Array.prototype.map() to return the new object using Array.prototype.reduce() to filter out the keys which were not supplied as the keys argument.
const reducedFilter = (data, keys, fn) =>
data.filter(fn).map(el =>
keys.reduce((acc, key) => {
acc[key] = el[key];
return acc;
}, {})
);Examples
const data = [
{
id: 1,
name: 'john',
age: 24
},
{
id: 2,
name: 'mike',
age: 50
}
];
reducedFilter(data, ['id', 'name'], item => item.age > 24); // [{ id: 2, name: 'mike'}]Applies a function against an accumulator and each element in the array (from left to right), returning an array of successively reduced values.
Use Array.prototype.reduce() to apply the given function to the given array, storing each new result.
const reduceSuccessive = (arr, fn, acc) =>
arr.reduce((res, val, i, arr) => (res.push(fn(res.slice(-1)[0], val, i, arr)), res), [acc]);Examples
reduceSuccessive([1, 2, 3, 4, 5, 6], (acc, val) => acc + val, 0); // [0, 1, 3, 6, 10, 15, 21]Returns the minimum/maximum value of an array, after applying the provided function to set comparing rule.
Use Array.prototype.reduce() in combination with the comparator function to get the appropriate element in the array.
You can omit the second parameter, comparator, to use the default one that returns the minimum element in the array.
const reduceWhich = (arr, comparator = (a, b) => a - b) =>
arr.reduce((a, b) => (comparator(a, b) >= 0 ? b : a));Examples
reduceWhich([1, 3, 2]); // 1
reduceWhich([1, 3, 2], (a, b) => b - a); // 3
reduceWhich(
[{ name: 'Tom', age: 12 }, { name: 'Jack', age: 18 }, { name: 'Lucy', age: 9 }],
(a, b) => a.age - b.age
); // {name: "Lucy", age: 9}Filters an array's values based on a predicate function, returning only values for which the predicate function returns true.
Use Array.prototype.filter() in combination with the predicate function, pred, to return only the values for which pred() returns true.
const reject = (pred, array) => array.filter((...args) => !pred(...args));Examples
reject(x => x % 2 === 0, [1, 2, 3, 4, 5]); // [1, 3, 5]
reject(word => word.length > 4, ['Apple', 'Pear', 'Kiwi', 'Banana']); // ['Pear', 'Kiwi']Removes elements from an array for which the given function returns false.
Use Array.prototype.filter() to find array elements that return truthy values and Array.prototype.reduce() to remove elements using Array.prototype.splice().
The func is invoked with three arguments (value, index, array).
const remove = (arr, func) =>
Array.isArray(arr)
? arr.filter(func).reduce((acc, val) => {
arr.splice(arr.indexOf(val), 1);
return acc.concat(val);
}, [])
: [];Examples
remove([1, 2, 3, 4], n => n % 2 === 0); // [2, 4]Returns a random element from an array.
Use Math.random() to generate a random number, multiply it by length and round it off to the nearest whole number using Math.floor().
This method also works with strings.
const sample = arr => arr[Math.floor(Math.random() * arr.length)];Examples
sample([3, 7, 9, 11]); // 9Gets n random elements at unique keys from array up to the size of array.
Shuffle the array using the Fisher-Yates algorithm.
Use Array.prototype.slice() to get the first n elements.
Omit the second argument, n to get only one element at random from the array.
const sampleSize = ([...arr], n = 1) => {
let m = arr.length;
while (m) {
const i = Math.floor(Math.random() * m--);
[arr[m], arr[i]] = [arr[i], arr[m]];
}
return arr.slice(0, n);
};Examples
sampleSize([1, 2, 3], 2); // [3,1]
sampleSize([1, 2, 3], 4); // [2,3,1]Has the same functionality as Array.prototype.splice(), but returning a new array instead of mutating the original array.
Use Array.prototype.slice() and Array.prototype.concat() to get a new array with the new contents after removing existing elements and/or adding new elements.
Omit the second argument, index, to start at 0.
Omit the third argument, delCount, to remove 0 elements.
Omit the fourth argument, elements, in order to not add any new elements.
const shank = (arr, index = 0, delCount = 0, ...elements) =>
arr
.slice(0, index)
.concat(elements)
.concat(arr.slice(index + delCount));Examples
const names = ['alpha', 'bravo', 'charlie'];
const namesAndDelta = shank(names, 1, 0, 'delta'); // [ 'alpha', 'delta', 'bravo', 'charlie' ]
const namesNoBravo = shank(names, 1, 1); // [ 'alpha', 'charlie' ]
console.log(names); // ['alpha', 'bravo', 'charlie']Randomizes the order of the values of an array, returning a new array.
Uses the Fisher-Yates algorithm to reorder the elements of the array.
const shuffle = ([...arr]) => {
let m = arr.length;
while (m) {
const i = Math.floor(Math.random() * m--);
[arr[m], arr[i]] = [arr[i], arr[m]];
}
return arr;
};Examples
const foo = [1, 2, 3];
shuffle(foo); // [2, 3, 1], foo = [1, 2, 3]Returns an array of elements that appear in both arrays.
Use Array.prototype.filter() to remove values that are not part of values, determined using Array.prototype.includes().
const similarity = (arr, values) => arr.filter(v => values.includes(v));Examples
similarity([1, 2, 3], [1, 2, 4]); // [1, 2]Returns the lowest index at which value should be inserted into array in order to maintain its sort order.
Check if the array is sorted in descending order (loosely).
Use Array.prototype.findIndex() to find the appropriate index where the element should be inserted.
const sortedIndex = (arr, n) => {
const isDescending = arr[0] > arr[arr.length - 1];
const index = arr.findIndex(el => (isDescending ? n >= el : n <= el));
return index === -1 ? arr.length : index;
};Examples
sortedIndex([5, 3, 2, 1], 4); // 1
sortedIndex([30, 50], 40); // 1Returns the lowest index at which value should be inserted into array in order to maintain its sort order, based on a provided iterator function.
Check if the array is sorted in descending order (loosely).
Use Array.prototype.findIndex() to find the appropriate index where the element should be inserted, based on the iterator function fn.
const sortedIndexBy = (arr, n, fn) => {
const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
const val = fn(n);
const index = arr.findIndex(el => (isDescending ? val >= fn(el) : val <= fn(el)));
return index === -1 ? arr.length : index;
};Examples
sortedIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 0Returns the highest index at which value should be inserted into array in order to maintain its sort order.
Check if the array is sorted in descending order (loosely).
Use Array.prototype.reverse() and Array.prototype.findIndex() to find the appropriate last index where the element should be inserted.
const sortedLastIndex = (arr, n) => {
const isDescending = arr[0] > arr[arr.length - 1];
const index = arr.reverse().findIndex(el => (isDescending ? n <= el : n >= el));
return index === -1 ? 0 : arr.length - index;
};Examples
sortedLastIndex([10, 20, 30, 30, 40], 30); // 4Returns the highest index at which value should be inserted into array in order to maintain its sort order, based on a provided iterator function.
Check if the array is sorted in descending order (loosely).
Use Array.prototype.map() to apply the iterator function to all elements of the array.
Use Array.prototype.reverse() and Array.prototype.findIndex() to find the appropriate last index where the element should be inserted, based on the provided iterator function.
const sortedLastIndexBy = (arr, n, fn) => {
const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
const val = fn(n);
const index = arr
.map(fn)
.reverse()
.findIndex(el => (isDescending ? val <= el : val >= el));
return index === -1 ? 0 : arr.length - index;
};Examples
sortedLastIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 1Performs stable sorting of an array, preserving the initial indexes of items when their values are the same. Does not mutate the original array, but returns a new array instead.
Use Array.prototype.map() to pair each element of the input array with its corresponding index.
Use Array.prototype.sort() and a compare function to sort the list, preserving their initial order if the items compared are equal.
Use Array.prototype.map() to convert back to the initial array items.
const stableSort = (arr, compare) =>
arr
.map((item, index) => ({ item, index }))
.sort((a, b) => compare(a.item, b.item) || a.index - b.index)
.map(({ item }) => item);Examples
const arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
const stable = stableSort(arr, () => 0); // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]Returns the symmetric difference between two arrays, without filtering out duplicate values.
Create a Set from each array, then use Array.prototype.filter() on each of them to only keep values not contained in the other.
const symmetricDifference = (a, b) => {
const sA = new Set(a),
sB = new Set(b);
return [...a.filter(x => !sB.has(x)), ...b.filter(x => !sA.has(x))];
};Examples
symmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
symmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 2, 3]Returns the symmetric difference between two arrays, after applying the provided function to each array element of both.
Create a Set by applying fn to each array's elements, then use Array.prototype.filter() on each of them to only keep values not contained in the other.
const symmetricDifferenceBy = (a, b, fn) => {
const sA = new Set(a.map(v => fn(v))),
sB = new Set(b.map(v => fn(v)));
return [...a.filter(x => !sB.has(fn(x))), ...b.filter(x => !sA.has(fn(x)))];
};Examples
symmetricDifferenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [ 1.2, 3.4 ]Returns the symmetric difference between two arrays, using a provided function as a comparator.
Use Array.prototype.filter() and Array.prototype.findIndex() to find the appropriate values.
const symmetricDifferenceWith = (arr, val, comp) => [
...arr.filter(a => val.findIndex(b => comp(a, b)) === -1),
...val.filter(a => arr.findIndex(b => comp(a, b)) === -1)
];Examples
symmetricDifferenceWith(
[1, 1.2, 1.5, 3, 0],
[1.9, 3, 0, 3.9],
(a, b) => Math.round(a) === Math.round(b)
); // [1, 1.2, 3.9]Returns all elements in an array except for the first one.
Return Array.prototype.slice(1) if the array's length is more than 1, otherwise, return the whole array.
const tail = arr => (arr.length > 1 ? arr.slice(1) : arr);Examples
tail([1, 2, 3]); // [2,3]
tail([1]); // [1]Returns an array with n elements removed from the beginning.
Use Array.prototype.slice() to create a slice of the array with n elements taken from the beginning.
const take = (arr, n = 1) => arr.slice(0, n);Examples
take([1, 2, 3], 5); // [1, 2, 3]
take([1, 2, 3], 0); // []Returns an array with n elements removed from the end.
Use Array.prototype.slice() to create a slice of the array with n elements taken from the end.
const takeRight = (arr, n = 1) => arr.slice(arr.length - n, arr.length);Examples
takeRight([1, 2, 3], 2); // [ 2, 3 ]
takeRight([1, 2, 3]); // [3]Removes elements from the end of an array until the passed function returns true. Returns the removed elements.
Loop through the array, using a Array.prototype.reduceRight() and accumulating elements while the function returns falsy value.
const takeRightWhile = (arr, func) =>
arr.reduceRight((acc, el) => (func(el) ? acc : [el, ...acc]), []);Examples
takeRightWhile([1, 2, 3, 4], n => n < 3); // [3, 4]Removes elements in an array until the passed function returns true. Returns the removed elements.
Loop through the array, using a for...of loop over Array.prototype.entries() until the returned value from the function is true.
Return the removed elements, using Array.prototype.slice().
const takeWhile = (arr, func) => {
for (const [i, val] of arr.entries()) if (func(val)) return arr.slice(0, i);
return arr;
};Examples
takeWhile([1, 2, 3, 4], n => n >= 3); // [1, 2]Reduces a given Array-like into a value hash (keyed data store).
Given an Iterable or Array-like structure, call Array.prototype.reduce.call() on the provided object to step over it and return an Object, keyed by the reference value.
const toHash = (object, key) =>
Array.prototype.reduce.call(
object,
(acc, data, index) => ((acc[!key ? index : data[key]] = data), acc),
{}
);Examples
toHash([4, 3, 2, 1]); // { 0: 4, 1: 3, 2: 2, 3: 1 }
toHash([{ a: 'label' }], 'a'); // { label: { a: 'label' } }
// A more in depth example:
let users = [{ id: 1, first: 'Jon' }, { id: 2, first: 'Joe' }, { id: 3, first: 'Moe' }];
let managers = [{ manager: 1, employees: [2, 3] }];
// We use function here because we want a bindable reference, but a closure referencing the hash would work, too.
managers.forEach(
manager =>
(manager.employees = manager.employees.map(function(id) {
return this[id];
}, toHash(users, 'id')))
);
managers; // [ { manager:1, employees: [ { id: 2, first: "Joe" }, { id: 3, first: "Moe" } ] } ]Returns every element that exists in any of the two arrays once.
Create a Set with all values of a and b and convert to an array.
const union = (a, b) => Array.from(new Set([...a, ...b]));Examples
union([1, 2, 3], [4, 3, 2]); // [1,2,3,4]Returns every element that exists in any of the two arrays once, after applying the provided function to each array element of both.
Create a Set by applying all fn to all values of a.
Create a Set from a and all elements in b whose value, after applying fn does not match a value in the previously created set.
Return the last set converted to an array.
const unionBy = (a, b, fn) => {
const s = new Set(a.map(fn));
return Array.from(new Set([...a, ...b.filter(x => !s.has(fn(x)))]));
};Examples
unionBy([2.1], [1.2, 2.3], Math.floor); // [2.1, 1.2]Returns every element that exists in any of the two arrays once, using a provided comparator function.
Create a Set with all values of a and values in b for which the comparator finds no matches in a, using Array.prototype.findIndex().
const unionWith = (a, b, comp) =>
Array.from(new Set([...a, ...b.filter(x => a.findIndex(y => comp(x, y)) === -1)]));Examples
unionWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0, 3.9], (a, b) => Math.round(a) === Math.round(b)); // [1, 1.2, 1.5, 3, 0, 3.9]Returns all unique values of an array.
Use ES6 Set and the ...rest operator to discard all duplicated values.
const uniqueElements = arr => [...new Set(arr)];Examples
uniqueElements([1, 2, 2, 3, 4, 4, 5]); // [1, 2, 3, 4, 5]Returns all unique values of an array, based on a provided comparator function.
Use Array.prototype.reduce() and Array.prototype.some() for an array containing only the first unique occurrence of each value, based on the comparator function, fn.
The comparator function takes two arguments: the values of the two elements being compared.
const uniqueElementsBy = (arr, fn) =>
arr.reduce((acc, v) => {
if (!acc.some(x => fn(v, x))) acc.push(v);
return acc;
}, []);Examples
uniqueElementsBy(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 1, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id == b.id
); // [ { id: 0, value: 'a' }, { id: 1, value: 'b' }, { id: 2, value: 'c' } ]Returns all unique values of an array, based on a provided comparator function, starting from the right.
Use Array.prototype.reduceRight() and Array.prototype.some() for an array containing only the last unique occurrence of each value, based on the comparator function, fn.
The comparator function takes two arguments: the values of the two elements being compared.
const uniqueElementsByRight = (arr, fn) =>
arr.reduceRight((acc, v) => {
if (!acc.some(x => fn(v, x))) acc.push(v);
return acc;
}, []);Examples
uniqueElementsByRight(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 1, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id == b.id
); // [ { id: 0, value: 'e' }, { id: 1, value: 'd' }, { id: 2, value: 'c' } ]Returns the unique symmetric difference between two arrays, not containing duplicate values from either array.
Use Array.prototype.filter() and Array.prototype.includes() on each array to remove values contained in the other, then create a Set from the results, removing duplicate values.
const uniqueSymmetricDifference = (a, b) => [
...new Set([...a.filter(v => !b.includes(v)), ...b.filter(v => !a.includes(v))])
];Examples
uniqueSymmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
uniqueSymmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 3]Creates an array of arrays, ungrouping the elements in an array produced by zip.
Use Math.max.apply() to get the longest subarray in the array, Array.prototype.map() to make each element an array.
Use Array.prototype.reduce() and Array.prototype.forEach() to map grouped values to individual arrays.
const unzip = arr =>
arr.reduce(
(acc, val) => (val.forEach((v, i) => acc[i].push(v)), acc),
Array.from({
length: Math.max(...arr.map(x => x.length))
}).map(x => [])
);Examples
unzip([['a', 1, true], ['b', 2, false]]); // [['a', 'b'], [1, 2], [true, false]]
unzip([['a', 1, true], ['b', 2]]); // [['a', 'b'], [1, 2], [true]]Creates an array of elements, ungrouping the elements in an array produced by zip and applying the provided function.
Use Math.max.apply() to get the longest subarray in the array, Array.prototype.map() to make each element an array.
Use Array.prototype.reduce() and Array.prototype.forEach() to map grouped values to individual arrays.
Use Array.prototype.map() and the spread operator (...) to apply fn to each individual group of elements.
const unzipWith = (arr, fn) =>
arr
.reduce(
(acc, val) => (val.forEach((v, i) => acc[i].push(v)), acc),
Array.from({
length: Math.max(...arr.map(x => x.length))
}).map(x => [])
)
.map(val => fn(...val));Examples
unzipWith([[1, 10, 100], [2, 20, 200]], (...args) => args.reduce((acc, v) => acc + v, 0)); // [3, 30, 300]Filters out the elements of an array, that have one of the specified values.
Use Array.prototype.filter() to create an array excluding(using !Array.includes()) all given values.
const without = (arr, ...args) => arr.filter(v => !args.includes(v));Examples
without([2, 1, 2, 3], 1, 2); // [3]Creates a new array out of the two supplied by creating each possible pair from the arrays.
Use Array.prototype.reduce(), Array.prototype.map() and Array.prototype.concat() to produce every possible pair from the elements of the two arrays and save them in an array.
const xProd = (a, b) => a.reduce((acc, x) => acc.concat(b.map(y => [x, y])), []);Examples
xProd([1, 2], ['a', 'b']); // [[1, 'a'], [1, 'b'], [2, 'a'], [2, 'b']]Creates an array of elements, grouped based on the position in the original arrays.
Use Math.max.apply() to get the longest array in the arguments.
Creates an array with that length as return value and use Array.from() with a map-function to create an array of grouped elements.
If lengths of the argument-arrays vary, undefined is used where no value could be found.
const zip = (...arrays) => {
const maxLength = Math.max(...arrays.map(x => x.length));
return Array.from({ length: maxLength }).map((_, i) => {
return Array.from({ length: arrays.length }, (_, k) => arrays[k][i]);
});
};Examples
zip(['a', 'b'], [1, 2], [true, false]); // [['a', 1, true], ['b', 2, false]]
zip(['a'], [1, 2], [true, false]); // [['a', 1, true], [undefined, 2, false]]Given an array of valid property identifiers and an array of values, return an object associating the properties to the values.
Since an object can have undefined values but not undefined property pointers, the array of properties is used to decide the structure of the resulting object using Array.prototype.reduce().
const zipObject = (props, values) =>
props.reduce((obj, prop, index) => ((obj[prop] = values[index]), obj), {});Examples
zipObject(['a', 'b', 'c'], [1, 2]); // {a: 1, b: 2, c: undefined}
zipObject(['a', 'b'], [1, 2, 3]); // {a: 1, b: 2}Creates an array of elements, grouped based on the position in the original arrays and using function as the last value to specify how grouped values should be combined.
Check if the last argument provided is a function.
Use Math.max() to get the longest array in the arguments.
Creates an array with that length as return value and use Array.from() with a map-function to create an array of grouped elements.
If lengths of the argument-arrays vary, undefined is used where no value could be found.
The function is invoked with the elements of each group (...group).
const zipWith = (...array) => {
const fn = typeof array[array.length - 1] === 'function' ? array.pop() : undefined;
return Array.from({ length: Math.max(...array.map(a => a.length)) }, (_, i) =>
fn ? fn(...array.map(a => a[i])) : array.map(a => a[i])
);
};Examples
zipWith([1, 2], [10, 20], [100, 200], (a, b, c) => a + b + c); // [111,222]
zipWith(
[1, 2, 3],
[10, 20],
[100, 200],
(a, b, c) => (a != null ? a : 'a') + (b != null ? b : 'b') + (c != null ? c : 'c')
); // [111, 222, '3bc']Converts the given array elements into <li> tags and appends them to the list of the given id.
Use Array.prototype.map(), document.querySelector(), and an anonymous inner closure to create a list of html tags.
const arrayToHtmlList = (arr, listID) =>
(el => (
(el = document.querySelector('#' + listID)),
(el.innerHTML += arr.map(item => `<li>${item}</li>`).join(''))
))();Examples
arrayToHtmlList(['item 1', 'item 2'], 'myListID');Returns true if the bottom of the page is visible, false otherwise.
Use scrollY, scrollHeight and clientHeight to determine if the bottom of the page is visible.
const bottomVisible = () =>
document.documentElement.clientHeight + window.scrollY >=
(document.documentElement.scrollHeight || document.documentElement.clientHeight);Examples
bottomVisible(); // trueCopy a string to the clipboard.
Only works as a result of user action (i.e. inside a click event listener).
Create a new <textarea> element, fill it with the supplied data and add it to the HTML document.
Use Selection.getRangeAt()to store the selected range (if any).
Use document.execCommand('copy') to copy to the clipboard.
Remove the <textarea> element from the HTML document.
Finally, use Selection().addRange() to recover the original selected range (if any).
const copyToClipboard = str => {
const el = document.createElement('textarea');
el.value = str;
el.setAttribute('readonly', '');
el.style.position = 'absolute';
el.style.left = '-9999px';
document.body.appendChild(el);
const selected =
document.getSelection().rangeCount > 0 ? document.getSelection().getRangeAt(0) : false;
el.select();
document.execCommand('copy');
document.body.removeChild(el);
if (selected) {
document.getSelection().removeAllRanges();
document.getSelection().addRange(selected);
}
};Examples
copyToClipboard('Lorem ipsum'); // 'Lorem ipsum' copied to clipboard.Creates a counter with the specified range, step and duration for the specified selector.
Check if step has the proper sign and change it accordingly.
Use setInterval() in combination with Math.abs() and Math.floor() to calculate the time between each new text draw.
Use document.querySelector().innerHTML to update the value of the selected element.
Omit the fourth parameter, step, to use a default step of 1.
Omit the fifth parameter, duration, to use a default duration of 2000ms.
const counter = (selector, start, end, step = 1, duration = 2000) => {
let current = start,
_step = (end - start) * step < 0 ? -step : step,
timer = setInterval(() => {
current += _step;
document.querySelector(selector).innerHTML = current;
if (current >= end) document.querySelector(selector).innerHTML = end;
if (current >= end) clearInterval(timer);
}, Math.abs(Math.floor(duration / (end - start))));
return timer;
};Examples
counter('#my-id', 1, 1000, 5, 2000); // Creates a 2-second timer for the element with id="my-id"Creates an element from a string (without appending it to the document). If the given string contains multiple elements, only the first one will be returned.
Use document.createElement() to create a new element.
Set its innerHTML to the string supplied as the argument.
Use ParentNode.firstElementChild to return the element version of the string.
const createElement = str => {
const el = document.createElement('div');
el.innerHTML = str;
return el.firstElementChild;
};Examples
const el = createElement(
`<div class="container">
<p>Hello!</p>
</div>`
);
console.log(el.className); // 'container'Creates a pub/sub (publish–subscribe) event hub with emit, on, and off methods.
Use Object.create(null) to create an empty hub object that does not inherit properties from Object.prototype.
For emit, resolve the array of handlers based on the event argument and then run each one with Array.prototype.forEach() by passing in the data as an argument.
For on, create an array for the event if it does not yet exist, then use Array.prototype.push() to add the handler
to the array.
For off, use Array.prototype.findIndex() to find the index of the handler in the event array and remove it using Array.prototype.splice().
const createEventHub = () => ({
hub: Object.create(null),
emit(event, data) {
(this.hub[event] || []).forEach(handler => handler(data));
},
on(event, handler) {
if (!this.hub[event]) this.hub[event] = [];
this.hub[event].push(handler);
},
off(event, handler) {
const i = (this.hub[event] || []).findIndex(h => h === handler);
if (i > -1) this.hub[event].splice(i, 1);
if (this.hub[event].length === 0) delete this.hub[event];
}
});Examples
const handler = data => console.log(data);
const hub = createEventHub();
let increment = 0;
// Subscribe: listen for different types of events
hub.on('message', handler);
hub.on('message', () => console.log('Message event fired'));
hub.on('increment', () => increment++);
// Publish: emit events to invoke all handlers subscribed to them, passing the data to them as an argument
hub.emit('message', 'hello world'); // logs 'hello world' and 'Message event fired'
hub.emit('message', { hello: 'world' }); // logs the object and 'Message event fired'
hub.emit('increment'); // `increment` variable is now 1
// Unsubscribe: stop a specific handler from listening to the 'message' event
hub.off('message', handler);Returns the current URL.
Use window.location.href to get current URL.
const currentURL = () => window.location.href;Examples
currentURL(); // 'https://google.com'Detects whether the website is being opened in a mobile device or a desktop/laptop.
Use a regular expression to test the navigator.userAgent property to figure out if the device is a mobile device or a desktop/laptop.
const detectDeviceType = () =>
/Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i.test(navigator.userAgent)
? 'Mobile'
: 'Desktop';Examples
detectDeviceType(); // "Mobile" or "Desktop"Returns true if the parent element contains the child element, false otherwise.
Check that parent is not the same element as child, use parent.contains(child) to check if the parent element contains the child element.
const elementContains = (parent, child) => parent !== child && parent.contains(child);Examples
elementContains(document.querySelector('head'), document.querySelector('title')); // true
elementContains(document.querySelector('body'), document.querySelector('body')); // falseReturns true if the element specified is visible in the viewport, false otherwise.
Use Element.getBoundingClientRect() and the window.inner(Width|Height) values
to determine if a given element is visible in the viewport.
Omit the second argument to determine if the element is entirely visible, or specify true to determine if
it is partially visible.
const elementIsVisibleInViewport = (el, partiallyVisible = false) => {
const { top, left, bottom, right } = el.getBoundingClientRect();
const { innerHeight, innerWidth } = window;
return partiallyVisible
? ((top > 0 && top < innerHeight) || (bottom > 0 && bottom < innerHeight)) &&
((left > 0 && left < innerWidth) || (right > 0 && right < innerWidth))
: top >= 0 && left >= 0 && bottom <= innerHeight && right <= innerWidth;
};Examples
// e.g. 100x100 viewport and a 10x10px element at position {top: -1, left: 0, bottom: 9, right: 10}
elementIsVisibleInViewport(el); // false - (not fully visible)
elementIsVisibleInViewport(el, true); // true - (partially visible)Encode a set of form elements as an object.
Use the FormData constructor to convert the HTML form to FormData, Array.from() to convert to an array.
Collect the object from the array, using Array.prototype.reduce().
const formToObject = form =>
Array.from(new FormData(form)).reduce(
(acc, [key, value]) => ({
...acc,
[key]: value
}),
{}
);Examples
formToObject(document.querySelector('#form')); // { email: 'test@email.com', name: 'Test Name' }Fetches all images from within an element and puts them into an array
Use Element.prototype.getElementsByTagName() to fetch all <img> elements inside the provided element, Array.prototype.map() to map every src attribute of their respective <img> element, then create a Set to eliminate duplicates and return the array.
const getImages = (el, includeDuplicates = false) => {
const images = [...el.getElementsByTagName('img')].map(img => img.getAttribute('src'));
return includeDuplicates ? images : [...new Set(images)];
};Examples
getImages(document, true); // ['image1.jpg', 'image2.png', 'image1.png', '...']
getImages(document, false); // ['image1.jpg', 'image2.png', '...']Returns the scroll position of the current page.
Use pageXOffset and pageYOffset if they are defined, otherwise scrollLeft and scrollTop.
You can omit el to use a default value of window.
const getScrollPosition = (el = window) => ({
x: el.pageXOffset !== undefined ? el.pageXOffset : el.scrollLeft,
y: el.pageYOffset !== undefined ? el.pageYOffset : el.scrollTop
});Examples
getScrollPosition(); // {x: 0, y: 200}Returns the value of a CSS rule for the specified element.
Use Window.getComputedStyle() to get the value of the CSS rule for the specified element.
const getStyle = (el, ruleName) => getComputedStyle(el)[ruleName];Examples
getStyle(document.querySelector('p'), 'font-size'); // '16px'Returns true if the element has the specified class, false otherwise.
Use element.classList.contains() to check if the element has the specified class.
const hasClass = (el, className) => el.classList.contains(className);Examples
hasClass(document.querySelector('p.special'), 'special'); // trueCreates a hash for a value using the SHA-256 algorithm. Returns a promise.
Use the SubtleCrypto API to create a hash for the given value.
const hashBrowser = val =>
crypto.subtle.digest('SHA-256', new TextEncoder('utf-8').encode(val)).then(h => {
let hexes = [],
view = new DataView(h);
for (let i = 0; i < view.byteLength; i += 4)
hexes.push(('00000000' + view.getUint32(i).toString(16)).slice(-8));
return hexes.join('');
});Examples
hashBrowser(JSON.stringify({ a: 'a', b: [1, 2, 3, 4], foo: { c: 'bar' } })).then(console.log); // '04aa106279f5977f59f9067fa9712afc4aedc6f5862a8defc34552d8c7206393'Hides all the elements specified.
Use NodeList.prototype.forEach() to apply display: none to each element specified.
const hide = (...el) => [...el].forEach(e => (e.style.display = 'none'));Examples
hide(document.querySelectorAll('img')); // Hides all <img> elements on the pageRedirects the page to HTTPS if its currently in HTTP. Also, pressing the back button doesn't take it back to the HTTP page as its replaced in the history.
Use location.protocol to get the protocol currently being used. If it's not HTTPS, use location.replace() to replace the existing page with the HTTPS version of the page. Use location.href to get the full address, split it with String.prototype.split() and remove the protocol part of the URL.
const httpsRedirect = () => {
if (location.protocol !== 'https:') location.replace('https://' + location.href.split('//')[1]);
};Examples
httpsRedirect(); // If you are on http://mydomain.com, you are redirected to https://mydomain.comInserts an HTML string after the end of the specified element.
Use el.insertAdjacentHTML() with a position of 'afterend' to parse htmlString and insert it after the end of el.
const insertAfter = (el, htmlString) => el.insertAdjacentHTML('afterend', htmlString);Examples
insertAfter(document.getElementById('myId'), '<p>after</p>'); // <div id="myId">...</div> <p>after</p>Inserts an HTML string before the start of the specified element.
Use el.insertAdjacentHTML() with a position of 'beforebegin' to parse htmlString and insert it before the start of el.
const insertBefore = (el, htmlString) => el.insertAdjacentHTML('beforebegin', htmlString);Examples
insertBefore(document.getElementById('myId'), '<p>before</p>'); // <p>before</p> <div id="myId">...</div>Returns true if the browser tab of the page is focused, false otherwise.
Use the Document.hidden property, introduced by the Page Visibility API to check if the browser tab of the page is visible or hidden.
const isBrowserTabFocused = () => !document.hidden;Examples
isBrowserTabFocused(); // trueConverts a NodeList to an array.
Use spread operator inside new array to convert a NodeList to an array.
const nodeListToArray = nodeList => [...nodeList];Examples
nodeListToArray(document.childNodes); // [ <!DOCTYPE html>, html ]Returns a new MutationObserver and runs the provided callback for each mutation on the specified element.
Use a MutationObserver to observe mutations on the given element.
Use Array.prototype.forEach() to run the callback for each mutation that is observed.
Omit the third argument, options, to use the default options (all true).
const observeMutations = (element, callback, options) => {
const observer = new MutationObserver(mutations => mutations.forEach(m => callback(m)));
observer.observe(
element,
Object.assign(
{
childList: true,
attributes: true,
attributeOldValue: true,
characterData: true,
characterDataOldValue: true,
subtree: true
},
options
)
);
return observer;
};Examples
const obs = observeMutations(document, console.log); // Logs all mutations that happen on the page
obs.disconnect(); // Disconnects the observer and stops logging mutations on the pageRemoves an event listener from an element.
Use EventTarget.removeEventListener() to remove an event listener from an element.
Omit the fourth argument opts to use false or specify it based on the options used when the event listener was added.
const off = (el, evt, fn, opts = false) => el.removeEventListener(evt, fn, opts);Examples
const fn = () => console.log('!');
document.body.addEventListener('click', fn);
off(document.body, 'click', fn); // no longer logs '!' upon clicking on the pageAdds an event listener to an element with the ability to use event delegation.
Use EventTarget.addEventListener() to add an event listener to an element. If there is a target property supplied to the options object, ensure the event target matches the target specified and then invoke the callback by supplying the correct this context.
Returns a reference to the custom delegator function, in order to be possible to use with off.
Omit opts to default to non-delegation behavior and event bubbling.
const on = (el, evt, fn, opts = {}) => {
const delegatorFn = e => e.target.matches(opts.target) && fn.call(e.target, e);
el.addEventListener(evt, opts.target ? delegatorFn : fn, opts.options || false);
if (opts.target) return delegatorFn;
};Examples
const fn = () => console.log('!');
on(document.body, 'click', fn); // logs '!' upon clicking the body
on(document.body, 'click', fn, { target: 'p' }); // logs '!' upon clicking a `p` element child of the body
on(document.body, 'click', fn, { options: true }); // use capturing instead of bubblingRun the callback whenever the user input type changes (mouse or touch). Useful for enabling/disabling code depending on the input device. This process is dynamic and works with hybrid devices (e.g. touchscreen laptops).
Use two event listeners. Assume mouse input initially and bind a touchstart event listener to the document.
On touchstart, add a mousemove event listener to listen for two consecutive mousemove events firing within 20ms, using performance.now().
Run the callback with the input type as an argument in either of these situations.
const onUserInputChange = callback => {
let type = 'mouse',
lastTime = 0;
const mousemoveHandler = () => {
const now = performance.now();
if (now - lastTime < 20)
(type = 'mouse'), callback(type), document.removeEventListener('mousemove', mousemoveHandler);
lastTime = now;
};
document.addEventListener('touchstart', () => {
if (type === 'touch') return;
(type = 'touch'), callback(type), document.addEventListener('mousemove', mousemoveHandler);
});
};Examples
onUserInputChange(type => {
console.log('The user is now using', type, 'as an input method.');
});Returns the prefixed version (if necessary) of a CSS property that the browser supports.
Use Array.prototype.findIndex() on an array of vendor prefix strings to test if document.body has one of them defined in its CSSStyleDeclaration object, otherwise return null.
Use String.prototype.charAt() and String.prototype.toUpperCase() to capitalize the property, which will be appended to the vendor prefix string.
const prefix = prop => {
const capitalizedProp = prop.charAt(0).toUpperCase() + prop.slice(1);
const prefixes = ['', 'webkit', 'moz', 'ms', 'o'];
const i = prefixes.findIndex(
prefix => typeof document.body.style[prefix ? prefix + capitalizedProp : prop] !== 'undefined'
);
return i !== -1 ? (i === 0 ? prop : prefixes[i] + capitalizedProp) : null;
};Examples
prefix('appearance'); // 'appearance' on a supported browser, otherwise 'webkitAppearance', 'mozAppearance', 'msAppearance' or 'oAppearance'Invokes the provided callback on each animation frame.
Use recursion.
Provided that running is true, continue invoking window.requestAnimationFrame() which invokes the provided callback.
Return an object with two methods start and stop to allow manual control of the recording.
Omit the second argument, autoStart, to implicitly call start when the function is invoked.
const recordAnimationFrames = (callback, autoStart = true) => {
let running = true,
raf;
const stop = () => {
running = false;
cancelAnimationFrame(raf);
};
const start = () => {
running = true;
run();
};
const run = () => {
raf = requestAnimationFrame(() => {
callback();
if (running) run();
});
};
if (autoStart) start();
return { start, stop };
};Examples
const cb = () => console.log('Animation frame fired');
const recorder = recordAnimationFrames(cb); // logs 'Animation frame fired' on each animation frame
recorder.stop(); // stops logging
recorder.start(); // starts again
const recorder2 = recordAnimationFrames(cb, false); // `start` needs to be explicitly called to begin recording framesRedirects to a specified URL.
Use window.location.href or window.location.replace() to redirect to url.
Pass a second argument to simulate a link click (true - default) or an HTTP redirect (false).
const redirect = (url, asLink = true) =>
asLink ? (window.location.href = url) : window.location.replace(url);Examples
redirect('https://google.com');Runs a function in a separate thread by using a Web Worker, allowing long running functions to not block the UI.
Create a new Worker using a Blob object URL, the contents of which should be the stringified version of the supplied function.
Immediately post the return value of calling the function back.
Return a promise, listening for onmessage and onerror events and resolving the data posted back from the worker, or throwing an error.
const runAsync = fn => {
const worker = new Worker(
URL.createObjectURL(new Blob([`postMessage((${fn})());`]), {
type: 'application/javascript; charset=utf-8'
})
);
return new Promise((res, rej) => {
worker.onmessage = ({ data }) => {
res(data), worker.terminate();
};
worker.onerror = err => {
rej(err), worker.terminate();
};
});
};Examples
const longRunningFunction = () => {
let result = 0;
for (let i = 0; i < 1000; i++)
for (let j = 0; j < 700; j++) for (let k = 0; k < 300; k++) result = result + i + j + k;
return result;
};
/*
NOTE: Since the function is running in a different context, closures are not supported.
The function supplied to `runAsync` gets stringified, so everything becomes literal.
All variables and functions must be defined inside.
*/
runAsync(longRunningFunction).then(console.log); // 209685000000
runAsync(() => 10 ** 3).then(console.log); // 1000
let outsideVariable = 50;
runAsync(() => typeof outsideVariable).then(console.log); // 'undefined'Smooth-scrolls to the top of the page.
Get distance from top using document.documentElement.scrollTop or document.body.scrollTop.
Scroll by a fraction of the distance from the top. Use window.requestAnimationFrame() to animate the scrolling.
const scrollToTop = () => {
const c = document.documentElement.scrollTop || document.body.scrollTop;
if (c > 0) {
window.requestAnimationFrame(scrollToTop);
window.scrollTo(0, c - c / 8);
}
};Examples
scrollToTop();Encode a set of form elements as a query string.
Use the FormData constructor to convert the HTML form to FormData, Array.from() to convert to an array, passing a map function as the second argument.
Use Array.prototype.map() and window.encodeURIComponent() to encode each field's value.
Use Array.prototype.join() with appropriate argumens to produce an appropriate query string.
const serializeForm = form =>
Array.from(new FormData(form), field => field.map(encodeURIComponent).join('=')).join('&');Examples
serializeForm(document.querySelector('#form')); // email=test%40email.com&name=Test%20NameSets the value of a CSS rule for the specified element.
Use element.style to set the value of the CSS rule for the specified element to val.
const setStyle = (el, ruleName, val) => (el.style[ruleName] = val);Examples
setStyle(document.querySelector('p'), 'font-size', '20px'); // The first <p> element on the page will have a font-size of 20pxShows all the elements specified.
Use the spread operator (...) and Array.prototype.forEach() to clear the display property for each element specified.
const show = (...el) => [...el].forEach(e => (e.style.display = ''));Examples
show(...document.querySelectorAll('img')); // Shows all <img> elements on the pageSmoothly scrolls the element on which it's called into the visible area of the browser window.
Use .scrollIntoView method to scroll the element.
Pass { behavior: 'smooth' } to .scrollIntoView so it scrolls smoothly.
const smoothScroll = element =>
document.querySelector(element).scrollIntoView({
behavior: 'smooth'
});Examples
smoothScroll('#fooBar'); // scrolls smoothly to the element with the id fooBar
smoothScroll('.fooBar'); // scrolls smoothly to the first element with a class of fooBarToggle a class for an element.
Use element.classList.toggle() to toggle the specified class for the element.
const toggleClass = (el, className) => el.classList.toggle(className);Examples
toggleClass(document.querySelector('p.special'), 'special'); // The paragraph will not have the 'special' class anymoreTriggers a specific event on a given element, optionally passing custom data.
Use new CustomEvent() to create an event from the specified eventType and details.
Use el.dispatchEvent() to trigger the newly created event on the given element.
Omit the third argument, detail, if you do not want to pass custom data to the triggered event.
const triggerEvent = (el, eventType, detail) =>
el.dispatchEvent(new CustomEvent(eventType, { detail }));Examples
triggerEvent(document.getElementById('myId'), 'click');
triggerEvent(document.getElementById('myId'), 'click', { username: 'bob' });Generates a UUID in a browser.
Use crypto API to generate a UUID, compliant with RFC4122 version 4.
const UUIDGeneratorBrowser = () =>
([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
(c ^ (crypto.getRandomValues(new Uint8Array(1))[0] & (15 >> (c / 4)))).toString(16)
);Examples
UUIDGeneratorBrowser(); // '7982fcfe-5721-4632-bede-6000885be57d'Gets the day of the year from a Date object.
Use new Date() and Date.prototype.getFullYear() to get the first day of the year as a Date object, subtract it from the provided date and divide with the milliseconds in each day to get the result.
Use Math.floor() to appropriately round the resulting day count to an integer.
const dayOfYear = date =>
Math.floor((date - new Date(date.getFullYear(), 0, 0)) / 1000 / 60 / 60 / 24);Examples
dayOfYear(new Date()); // 272Returns the human readable format of the given number of milliseconds.
Divide ms with the appropriate values to obtain the appropriate values for day, hour, minute, second and millisecond.
Use Object.entries() with Array.prototype.filter() to keep only non-zero values.
Use Array.prototype.map() to create the string for each value, pluralizing appropriately.
Use String.prototype.join(', ') to combine the values into a string.
const formatDuration = ms => {
if (ms < 0) ms = -ms;
const time = {
day: Math.floor(ms / 86400000),
hour: Math.floor(ms / 3600000) % 24,
minute: Math.floor(ms / 60000) % 60,
second: Math.floor(ms / 1000) % 60,
millisecond: Math.floor(ms) % 1000
};
return Object.entries(time)
.filter(val => val[1] !== 0)
.map(([key, val]) => `${val} ${key}${val !== 1 ? 's' : ''}`)
.join(', ');
};Examples
formatDuration(1001); // '1 second, 1 millisecond'
formatDuration(34325055574); // '397 days, 6 hours, 44 minutes, 15 seconds, 574 milliseconds'Returns a string of the form HH:MM:SS from a Date object.
Use Date.prototype.toTimeString() and String.prototype.slice() to get the HH:MM:SS part of a given Date object.
const getColonTimeFromDate = date => date.toTimeString().slice(0, 8);Examples
getColonTimeFromDate(new Date()); // "08:38:00"Returns the difference (in days) between two dates.
Calculate the difference (in days) between two Date objects.
const getDaysDiffBetweenDates = (dateInitial, dateFinal) =>
(dateFinal - dateInitial) / (1000 * 3600 * 24);Examples
getDaysDiffBetweenDates(new Date('2017-12-13'), new Date('2017-12-22')); // 9Converts an integer to a suffixed string, adding am or pm based on its value.
Use the modulo operator (%) and conditional checks to transform an integer to a stringified 12-hour format with meridiem suffix.
const getMeridiemSuffixOfInteger = num =>
num === 0 || num === 24
? 12 + 'am'
: num === 12
? 12 + 'pm'
: num < 12
? (num % 12) + 'am'
: (num % 12) + 'pm';Examples
getMeridiemSuffixOfInteger(0); // "12am"
getMeridiemSuffixOfInteger(11); // "11am"
getMeridiemSuffixOfInteger(13); // "1pm"
getMeridiemSuffixOfInteger(25); // "1pm"Check if a date is after another date.
Use the greater than operator (>) to check if the first date comes after the second one.
const isAfterDate = (dateA, dateB) => dateA > dateB;Examples
isAfterDate(new Date(2010, 10, 21), new Date(2010, 10, 20)); // trueCheck if a date is before another date.
Use the less than operator (<) to check if the first date comes before the second one.
const isBeforeDate = (dateA, dateB) => dateA < dateB;Examples
isBeforeDate(new Date(2010, 10, 20), new Date(2010, 10, 21)); // trueCheck if a date is the same as another date.
Use Date.prototype.toISOString() and strict equality checking (===) to check if the first date is the same as the second one.
const isSameDate = (dateA, dateB) => dateA.toISOString() === dateB.toISOString();Examples
isSameDate(new Date(2010, 10, 20), new Date(2010, 10, 20)); // trueResults in a boolean representation of a specific date.
Pass the specific date object firstly.
Use Date.getDay() to check weekday by using a modulo operator and then returning a boolean.
const isWeekday = (t = new Date()) => {
return t.getDay() % 6 !== 0;
};Examples
isWeekday(); // true (if current date is 2019-07-19)Results in a boolean representation of a specific date.
Pass the specific date object firstly.
Use Date.getDay() to check weekend based on the day being returned as 0 - 6 using a modulo operation then return a boolean.
const isWeekend = (t = new Date()) => {
return t.getDay() % 6 === 0;
};Examples
isWeekend(); // 2018-10-19 (if current date is 2018-10-18)Returns the maximum of the given dates.
Use the ES6 spread syntax with Math.max to find the maximum date value, new Date() to convert it to a Date object.
const maxDate = dates => new Date(Math.max(...dates));Examples
const array = [
new Date(2017, 4, 13),
new Date(2018, 2, 12),
new Date(2016, 0, 10),
new Date(2016, 0, 9)
];
maxDate(array); // 2018-03-11T22:00:00.000ZReturns the minimum of the given dates.
Use the ES6 spread syntax to find the minimum date value, new Date() to convert it to a Date object.
const minDate = dates => new Date(Math.min(...dates));Examples
const array = [
new Date(2017, 4, 13),
new Date(2018, 2, 12),
new Date(2016, 0, 10),
new Date(2016, 0, 9)
];
minDate(array); // 2016-01-08T22:00:00.000ZResults in a string representation of tomorrow's date.
Use new Date() to get the current date, increment by one using Date.getDate() and set the value to the result using Date.setDate().
Use Date.prototype.toISOString() to return a string in yyyy-mm-dd format.
const tomorrow = () => {
let t = new Date();
t.setDate(t.getDate() + 1);
return t.toISOString().split('T')[0];
};Examples
tomorrow(); // 2018-10-19 (if current date is 2018-10-18)Results in a string representation of yesterday's date.
Use new Date() to get the current date, decrement by one using Date.getDate() and set the value to the result using Date.setDate().
Use Date.prototype.toISOString() to return a string in yyyy-mm-dd format.
const yesterday = () => {
let t = new Date();
t.setDate(t.getDate() - 1);
return t.toISOString().split('T')[0];
};Examples
yesterday(); // 2018-10-17 (if current date is 2018-10-18)Attempts to invoke a function with the provided arguments, returning either the result or the caught error object.
Use a try... catch block to return either the result of the function or an appropriate error.
const attempt = (fn, ...args) => {
try {
return fn(...args);
} catch (e) {
return e instanceof Error ? e : new Error(e);
}
};Examples
var elements = attempt(function(selector) {
return document.querySelectorAll(selector);
}, '>_>');
if (elements instanceof Error) elements = []; // elements = []Creates a function that invokes fn with a given context, optionally adding any additional supplied parameters to the beginning of the arguments.
Return a function that uses Function.prototype.apply() to apply the given context to fn.
Use Array.prototype.concat() to prepend any additional supplied parameters to the arguments.
const bind = (fn, context, ...boundArgs) => (...args) => fn.apply(context, [...boundArgs, ...args]);Examples
function greet(greeting, punctuation) {
return greeting + ' ' + this.user + punctuation;
}
const freddy = { user: 'fred' };
const freddyBound = bind(greet, freddy);
console.log(freddyBound('hi', '!')); // 'hi fred!'Creates a function that invokes the method at a given key of an object, optionally adding any additional supplied parameters to the beginning of the arguments.
Return a function that uses Function.prototype.apply() to bind context[fn] to context.
Use the spread operator (...) to prepend any additional supplied parameters to the arguments.
const bindKey = (context, fn, ...boundArgs) => (...args) =>
context[fn].apply(context, [...boundArgs, ...args]);Examples
const freddy = {
user: 'fred',
greet: function(greeting, punctuation) {
return greeting + ' ' + this.user + punctuation;
}
};
const freddyBound = bindKey(freddy, 'greet');
console.log(freddyBound('hi', '!')); // 'hi fred!'Chains asynchronous functions.
Loop through an array of functions containing asynchronous events, calling next when each asynchronous event has completed.
const chainAsync = fns => {
let curr = 0;
const last = fns[fns.length - 1];
const next = () => {
const fn = fns[curr++];
fn === last ? fn() : fn(next);
};
next();
};Examples
chainAsync([
next => {
console.log('0 seconds');
setTimeout(next, 1000);
},
next => {
console.log('1 second');
setTimeout(next, 1000);
},
() => {
console.log('2 second');
}
]);Given a predicate function and a prop string, this curried function will then take an object to inspect by calling the property and passing it to the predicate.
Summon prop on obj, pass it to a provided predicate function and return a masked boolean.
const checkProp = (predicate, prop) => obj => !!predicate(obj[prop]);Examples
const lengthIs4 = checkProp(l => l === 4, 'length');
lengthIs4([]); // false
lengthIs4([1,2,3,4]); // true
lengthIs4(new Set([1,2,3,4])); // false (Set uses Size, not length)
const session = { user: {} };
const validUserSession = checkProps(u => u.active && !u.disabled, 'user');
validUserSession(session); // false
session.user.active = true;
validUserSession(session); // true
const noLength(l => l === undefined, 'length');
noLength([]); // false
noLength({}); // true
noLength(new Set()); // truePerforms right-to-left function composition.
Use Array.prototype.reduce() to perform right-to-left function composition.
The last (rightmost) function can accept one or more arguments; the remaining functions must be unary.
const compose = (...fns) => fns.reduce((f, g) => (...args) => f(g(...args)));Examples
const add5 = x => x + 5;
const multiply = (x, y) => x * y;
const multiplyAndAdd5 = compose(
add5,
multiply
);
multiplyAndAdd5(5, 2); // 15Performs left-to-right function composition.
Use Array.prototype.reduce() to perform left-to-right function composition.
The first (leftmost) function can accept one or more arguments; the remaining functions must be unary.
const composeRight = (...fns) => fns.reduce((f, g) => (...args) => g(f(...args)));Examples
const add = (x, y) => x + y;
const square = x => x * x;
const addAndSquare = composeRight(add, square);
addAndSquare(1, 2); // 9Accepts a converging function and a list of branching functions and returns a function that applies each branching function to the arguments and the results of the branching functions are passed as arguments to the converging function.
Use Array.prototype.map() and Function.prototype.apply() to apply each function to the given arguments.
Use the spread operator (...) to call coverger with the results of all other functions.
const converge = (converger, fns) => (...args) => converger(...fns.map(fn => fn.apply(null, args)));Examples
const average = converge((a, b) => a / b, [
arr => arr.reduce((a, v) => a + v, 0),
arr => arr.length
]);
average([1, 2, 3, 4, 5, 6, 7]); // 4Curries a function.
Use recursion.
If the number of provided arguments (args) is sufficient, call the passed function fn.
Otherwise, return a curried function fn that expects the rest of the arguments.
If you want to curry a function that accepts a variable number of arguments (a variadic function, e.g. Math.min()), you can optionally pass the number of arguments to the second parameter arity.
const curry = (fn, arity = fn.length, ...args) =>
arity <= args.length ? fn(...args) : curry.bind(null, fn, arity, ...args);Examples
curry(Math.pow)(2)(10); // 1024
curry(Math.min, 3)(10)(50)(2); // 2Creates a debounced function that delays invoking the provided function until at least ms milliseconds have elapsed since the last time it was invoked.
Each time the debounced function is invoked, clear the current pending timeout with clearTimeout() and use setTimeout() to create a new timeout that delays invoking the function until at least ms milliseconds has elapsed. Use Function.prototype.apply() to apply the this context to the function and provide the necessary arguments.
Omit the second argument, ms, to set the timeout at a default of 0 ms.
const debounce = (fn, ms = 0) => {
let timeoutId;
return function(...args) {
clearTimeout(timeoutId);
timeoutId = setTimeout(() => fn.apply(this, args), ms);
};
};Examples
window.addEventListener(
'resize',
debounce(() => {
console.log(window.innerWidth);
console.log(window.innerHeight);
}, 250)
); // Will log the window dimensions at most every 250msDefers invoking a function until the current call stack has cleared.
Use setTimeout() with a timeout of 1ms to add a new event to the browser event queue and allow the rendering engine to complete its work. Use the spread (...) operator to supply the function with an arbitrary number of arguments.
const defer = (fn, ...args) => setTimeout(fn, 1, ...args);Examples
// Example A:
defer(console.log, 'a'), console.log('b'); // logs 'b' then 'a'
// Example B:
document.querySelector('#someElement').innerHTML = 'Hello';
longRunningFunction(); // Browser will not update the HTML until this has finished
defer(longRunningFunction); // Browser will update the HTML then run the functionInvokes the provided function after wait milliseconds.
Use setTimeout() to delay execution of fn.
Use the spread (...) operator to supply the function with an arbitrary number of arguments.
const delay = (fn, wait, ...args) => setTimeout(fn, wait, ...args);Examples
delay(
function(text) {
console.log(text);
},
1000,
'later'
); // Logs 'later' after one second.Logs the name of a function.
Use console.debug() and the name property of the passed method to log the method's name to the debug channel of the console.
const functionName = fn => (console.debug(fn.name), fn);Examples
functionName(Math.max); // max (logged in debug channel of console)Returns the number of times a function executed per second.
hz is the unit for hertz, the unit of frequency defined as one cycle per second.
Use performance.now() to get the difference in milliseconds before and after the iteration loop to calculate the time elapsed executing the function iterations times.
Return the number of cycles per second by converting milliseconds to seconds and dividing it by the time elapsed.
Omit the second argument, iterations, to use the default of 100 iterations.
const hz = (fn, iterations = 100) => {
const before = performance.now();
for (let i = 0; i < iterations; i++) fn();
return (1000 * iterations) / (performance.now() - before);
};Examples
// 10,000 element array
const numbers = Array(10000)
.fill()
.map((_, i) => i);
// Test functions with the same goal: sum up the elements in the array
const sumReduce = () => numbers.reduce((acc, n) => acc + n, 0);
const sumForLoop = () => {
let sum = 0;
for (let i = 0; i < numbers.length; i++) sum += numbers[i];
return sum;
};
// `sumForLoop` is nearly 10 times faster
Math.round(hz(sumReduce)); // 572
Math.round(hz(sumForLoop)); // 4784Returns the memoized (cached) function.
Create an empty cache by instantiating a new Map object.
Return a function which takes a single argument to be supplied to the memoized function by first checking if the function's output for that specific input value is already cached, or store and return it if not. The function keyword must be used in order to allow the memoized function to have its this context changed if necessary.
Allow access to the cache by setting it as a property on the returned function.
const memoize = fn => {
const cache = new Map();
const cached = function(val) {
return cache.has(val) ? cache.get(val) : cache.set(val, fn.call(this, val)) && cache.get(val);
};
cached.cache = cache;
return cached;
};Examples
// See the `anagrams` snippet.
const anagramsCached = memoize(anagrams);
anagramsCached('javascript'); // takes a long time
anagramsCached('javascript'); // returns virtually instantly since it's now cached
console.log(anagramsCached.cache); // The cached anagrams mapNegates a predicate function.
Take a predicate function and apply the not operator (!) to it with its arguments.
const negate = func => (...args) => !func(...args);Examples
[1, 2, 3, 4, 5, 6].filter(negate(n => n % 2 === 0)); // [ 1, 3, 5 ]Ensures a function is called only once.
Utilizing a closure, use a flag, called, and set it to true once the function is called for the first time, preventing it from being called again. In order to allow the function to have its this context changed (such as in an event listener), the function keyword must be used, and the supplied function must have the context applied.
Allow the function to be supplied with an arbitrary number of arguments using the rest/spread (...) operator.
const once = fn => {
let called = false;
return function(...args) {
if (called) return;
called = true;
return fn.apply(this, args);
};
};Examples
const startApp = function(event) {
console.log(this, event); // document.body, MouseEvent
};
document.body.addEventListener('click', once(startApp)); // only runs `startApp` once upon clickCreates a function that invokes fn with partials prepended to the arguments it receives.
Use the spread operator (...) to prepend partials to the list of arguments of fn.
const partial = (fn, ...partials) => (...args) => fn(...partials, ...args);Examples
const greet = (greeting, name) => greeting + ' ' + name + '!';
const greetHello = partial(greet, 'Hello');
greetHello('John'); // 'Hello John!'Creates a function that invokes fn with partials appended to the arguments it receives.
Use the spread operator (...) to append partials to the list of arguments of fn.
const partialRight = (fn, ...partials) => (...args) => fn(...args, ...partials);Examples
const greet = (greeting, name) => greeting + ' ' + name + '!';
const greetJohn = partialRight(greet, 'John');
greetJohn('Hello'); // 'Hello John!'Runs an array of promises in series.
Use Array.prototype.reduce() to create a promise chain, where each promise returns the next promise when resolved.
const runPromisesInSeries = ps => ps.reduce((p, next) => p.then(next), Promise.resolve());Examples
const delay = d => new Promise(r => setTimeout(r, d));
runPromisesInSeries([() => delay(1000), () => delay(2000)]); // Executes each promise sequentially, taking a total of 3 seconds to completeDelays the execution of an asynchronous function.
Delay executing part of an async function, by putting it to sleep, returning a Promise.
const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));Examples
async function sleepyWork() {
console.log("I'm going to sleep for 1 second.");
await sleep(1000);
console.log('I woke up after 1 second.');
}Creates a throttled function that only invokes the provided function at most once per every wait milliseconds
Use setTimeout() and clearTimeout() to throttle the given method, fn.
Use Function.prototype.apply() to apply the this context to the function and provide the necessary arguments.
Use Date.now() to keep track of the last time the throttled function was invoked.
Omit the second argument, wait, to set the timeout at a default of 0 ms.
const throttle = (fn, wait) => {
let inThrottle, lastFn, lastTime;
return function() {
const context = this,
args = arguments;
if (!inThrottle) {
fn.apply(context, args);
lastTime = Date.now();
inThrottle = true;
} else {
clearTimeout(lastFn);
lastFn = setTimeout(function() {
if (Date.now() - lastTime >= wait) {
fn.apply(context, args);
lastTime = Date.now();
}
}, Math.max(wait - (Date.now() - lastTime), 0));
}
};
};Examples
window.addEventListener(
'resize',
throttle(function(evt) {
console.log(window.innerWidth);
console.log(window.innerHeight);
}, 250)
); // Will log the window dimensions at most every 250msIterates over a callback n times
Use Function.call() to call fn n times or until it returns false.
Omit the last argument, context, to use an undefined object (or the global object in non-strict mode).
const times = (n, fn, context = undefined) => {
let i = 0;
while (fn.call(context, i) !== false && ++i < n) {}
};Examples
var output = '';
times(5, i => (output += i));
console.log(output); // 01234Uncurries a function up to depth n.
Return a variadic function.
Use Array.prototype.reduce() on the provided arguments to call each subsequent curry level of the function.
If the length of the provided arguments is less than n throw an error.
Otherwise, call fn with the proper amount of arguments, using Array.prototype.slice(0, n).
Omit the second argument, n, to uncurry up to depth 1.
const uncurry = (fn, n = 1) => (...args) => {
const next = acc => args => args.reduce((x, y) => x(y), acc);
if (n > args.length) throw new RangeError('Arguments too few!');
return next(fn)(args.slice(0, n));
};Examples
const add = x => y => z => x + y + z;
const uncurriedAdd = uncurry(add, 3);
uncurriedAdd(1, 2, 3); // 6Builds an array, using an iterator function and an initial seed value.
Use a while loop and Array.prototype.push() to call the function repeatedly until it returns false.
The iterator function accepts one argument (seed) and must always return an array with two elements ([value, nextSeed]) or false to terminate.
const unfold = (fn, seed) => {
let result = [],
val = [null, seed];
while ((val = fn(val[1]))) result.push(val[0]);
return result;
};Examples
var f = n => (n > 50 ? false : [-n, n + 10]);
unfold(f, 10); // [-10, -20, -30, -40, -50]Tests a value, x, against a predicate function. If true, return fn(x). Else, return x.
Return a function expecting a single value, x, that returns the appropriate value based on pred.
const when = (pred, whenTrue) => x => (pred(x) ? whenTrue(x) : x);Examples
const doubleEvenNumbers = when(x => x % 2 === 0, x => x * 2);
doubleEvenNumbers(2); // 4
doubleEvenNumbers(1); // 1Checks if two numbers are approximately equal to each other.
Use Math.abs() to compare the absolute difference of the two values to epsilon.
Omit the third parameter, epsilon, to use a default value of 0.001.
const approximatelyEqual = (v1, v2, epsilon = 0.001) => Math.abs(v1 - v2) < epsilon;Examples
approximatelyEqual(Math.PI / 2.0, 1.5708); // trueReturns the average of two or more numbers.
Use Array.prototype.reduce() to add each value to an accumulator, initialized with a value of 0, divide by the length of the array.
const average = (...nums) => nums.reduce((acc, val) => acc + val, 0) / nums.length;Examples
average(...[1, 2, 3]); // 2
average(1, 2, 3); // 2Returns the average of an array, after mapping each element to a value using the provided function.
Use Array.prototype.map() to map each element to the value returned by fn, Array.prototype.reduce() to add each value to an accumulator, initialized with a value of 0, divide by the length of the array.
const averageBy = (arr, fn) =>
arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => acc + val, 0) /
arr.length;Examples
averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 5
averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 5Evaluates the binomial coefficient of two integers n and k.
Use Number.isNaN() to check if any of the two values is NaN.
Check if k is less than 0, greater than or equal to n, equal to 1 or n - 1 and return the appropriate result.
Check if n - k is less than k and switch their values accordingly.
Loop from 2 through k and calculate the binomial coefficient.
Use Math.round() to account for rounding errors in the calculation.
const binomialCoefficient = (n, k) => {
if (Number.isNaN(n) || Number.isNaN(k)) return NaN;
if (k < 0 || k > n) return 0;
if (k === 0 || k === n) return 1;
if (k === 1 || k === n - 1) return n;
if (n - k < k) k = n - k;
let res = n;
for (let j = 2; j <= k; j++) res *= (n - j + 1) / j;
return Math.round(res);
};Examples
binomialCoefficient(8, 2); // 28Clamps num within the inclusive range specified by the boundary values a and b.
If num falls within the range, return num.
Otherwise, return the nearest number in the range.
const clampNumber = (num, a, b) => Math.max(Math.min(num, Math.max(a, b)), Math.min(a, b));Examples
clampNumber(2, 3, 5); // 3
clampNumber(1, -1, -5); // -1Converts an angle from degrees to radians.
Use Math.PI and the degree to radian formula to convert the angle from degrees to radians.
const degreesToRads = deg => (deg * Math.PI) / 180.0;Examples
degreesToRads(90.0); // ~1.5708Converts a number to an array of digits.
Convert the number to a string, using the spread operator (...) to build an array.
Use Array.prototype.map() and parseInt() to transform each value to an integer.
const digitize = n => [...`${n}`].map(i => parseInt(i));Examples
digitize(123); // [1, 2, 3]Returns the distance between two points.
Use Math.hypot() to calculate the Euclidean distance between two points.
const distance = (x0, y0, x1, y1) => Math.hypot(x1 - x0, y1 - y0);Examples
distance(1, 1, 2, 3); // 2.23606797749979Computes the new ratings between two or more opponents using the Elo rating system. It takes an array of pre-ratings and returns an array containing post-ratings. The array should be ordered from best performer to worst performer (winner -> loser).
Use the exponent ** operator and math operators to compute the expected score (chance of winning).
of each opponent and compute the new rating for each.
Loop through the ratings, using each permutation to compute the post-Elo rating for each player in a pairwise fashion.
Omit the second argument to use the default kFactor of 32.
const elo = ([...ratings], kFactor = 32, selfRating) => {
const [a, b] = ratings;
const expectedScore = (self, opponent) => 1 / (1 + 10 ** ((opponent - self) / 400));
const newRating = (rating, i) =>
(selfRating || rating) + kFactor * (i - expectedScore(i ? a : b, i ? b : a));
if (ratings.length === 2) return [newRating(a, 1), newRating(b, 0)];
for (let i = 0, len = ratings.length; i < len; i++) {
let j = i;
while (j < len - 1) {
j++;
[ratings[i], ratings[j]] = elo([ratings[i], ratings[j]], kFactor);
}
}
return ratings;
};Examples
// Standard 1v1s
elo([1200, 1200]); // [1216, 1184]
elo([1200, 1200], 64); // [1232, 1168]
// 4 player FFA, all same rank
elo([1200, 1200, 1200, 1200]).map(Math.round); // [1246, 1215, 1185, 1154]
/*
For teams, each rating can adjusted based on own team's average rating vs.
average rating of opposing team, with the score being added to their
own individual rating by supplying it as the third argument.
*/Calculates the factorial of a number.
Use recursion.
If n is less than or equal to 1, return 1.
Otherwise, return the product of n and the factorial of n - 1.
Throws an exception if n is a negative number.
const factorial = n =>
n < 0
? (() => {
throw new TypeError('Negative numbers are not allowed!');
})()
: n <= 1
? 1
: n * factorial(n - 1);Examples
factorial(6); // 720Generates an array, containing the Fibonacci sequence, up until the nth term.
Create an empty array of the specific length, initializing the first two values (0 and 1).
Use Array.prototype.reduce() to add values into the array, using the sum of the last two values, except for the first two.
const fibonacci = n =>
Array.from({ length: n }).reduce(
(acc, val, i) => acc.concat(i > 1 ? acc[i - 1] + acc[i - 2] : i),
[]
);Examples
fibonacci(6); // [0, 1, 1, 2, 3, 5]Calculates the greatest common divisor between two or more numbers/arrays.
The inner _gcd function uses recursion.
Base case is when y equals 0. In this case, return x.
Otherwise, return the GCD of y and the remainder of the division x/y.
const gcd = (...arr) => {
const _gcd = (x, y) => (!y ? x : gcd(y, x % y));
return [...arr].reduce((a, b) => _gcd(a, b));
};Examples
gcd(8, 36); // 4
gcd(...[12, 8, 32]); // 4Initializes an array containing the numbers in the specified range where start and end are inclusive and the ratio between two terms is step.
Returns an error if step equals 1.
Use Array.from(), Math.log() and Math.floor() to create an array of the desired length, Array.prototype.map() to fill with the desired values in a range.
Omit the second argument, start, to use a default value of 1.
Omit the third argument, step, to use a default value of 2.
const geometricProgression = (end, start = 1, step = 2) =>
Array.from({ length: Math.floor(Math.log(end / start) / Math.log(step)) + 1 }).map(
(v, i) => start * step ** i
);Examples
geometricProgression(256); // [1, 2, 4, 8, 16, 32, 64, 128, 256]
geometricProgression(256, 3); // [3, 6, 12, 24, 48, 96, 192]
geometricProgression(256, 1, 4); // [1, 4, 16, 64, 256]Calculates the Hamming distance between two values.
Use XOR operator (^) to find the bit difference between the two numbers, convert to a binary string using toString(2).
Count and return the number of 1s in the string, using match(/1/g).
const hammingDistance = (num1, num2) => ((num1 ^ num2).toString(2).match(/1/g) || '').length;Examples
hammingDistance(2, 3); // 1Checks if the given number falls within the given range.
Use arithmetic comparison to check if the given number is in the specified range.
If the second parameter, end, is not specified, the range is considered to be from 0 to start.
const inRange = (n, start, end = null) => {
if (end && start > end) [end, start] = [start, end];
return end == null ? n >= 0 && n < start : n >= start && n < end;
};Examples
inRange(3, 2, 5); // true
inRange(3, 4); // true
inRange(2, 3, 5); // false
inRange(3, 2); // falseChecks if the first numeric argument is divisible by the second one.
Use the modulo operator (%) to check if the remainder is equal to 0.
const isDivisible = (dividend, divisor) => dividend % divisor === 0;Examples
isDivisible(6, 3); // trueReturns true if the given number is even, false otherwise.
Checks whether a number is odd or even using the modulo (%) operator.
Returns true if the number is even, false if the number is odd.
const isEven = num => num % 2 === 0;Examples
isEven(3); // falseChecks if the given value is equal to negative zero (-0).
Checks whether a passed value is equal to 0 and if 1 divided by the value equals -Infinity.
const isNegativeZero = val => val === 0 && 1 / val === -Infinity;Examples
isNegativeZero(-0); // true
isNegativeZero(0); // falseReturns true if the given number is odd, false otherwise.
Checks whether a number is odd or even using the modulo (%) operator.
Returns true if the number is odd, false if the number is even.
const isOdd = num => num % 2 === 1;Examples
isOdd(3); // trueChecks if the provided integer is a prime number.
Check numbers from 2 to the square root of the given number.
Return false if any of them divides the given number, else return true, unless the number is less than 2.
const isPrime = num => {
const boundary = Math.floor(Math.sqrt(num));
for (var i = 2; i <= boundary; i++) if (num % i === 0) return false;
return num >= 2;
};Examples
isPrime(11); // trueReturns the least common multiple of two or more numbers.
Use the greatest common divisor (GCD) formula and the fact that lcm(x,y) = x * y / gcd(x,y) to determine the least common multiple.
The GCD formula uses recursion.
const lcm = (...arr) => {
const gcd = (x, y) => (!y ? x : gcd(y, x % y));
const _lcm = (x, y) => (x * y) / gcd(x, y);
return [...arr].reduce((a, b) => _lcm(a, b));
};Examples
lcm(12, 7); // 84
lcm(...[1, 3, 4, 5]); // 60Implementation of the Luhn Algorithm used to validate a variety of identification numbers, such as credit card numbers, IMEI numbers, National Provider Identifier numbers etc.
Use String.prototype.split(''), Array.prototype.reverse() and Array.prototype.map() in combination with parseInt() to obtain an array of digits.
Use Array.prototype.splice(0,1) to obtain the last digit.
Use Array.prototype.reduce() to implement the Luhn Algorithm.
Return true if sum is divisible by 10, false otherwise.
const luhnCheck = num => {
let arr = (num + '')
.split('')
.reverse()
.map(x => parseInt(x));
let lastDigit = arr.splice(0, 1)[0];
let sum = arr.reduce((acc, val, i) => (i % 2 !== 0 ? acc + val : acc + ((val * 2) % 9) || 9), 0);
sum += lastDigit;
return sum % 10 === 0;
};Examples
luhnCheck('4485275742308327'); // true
luhnCheck(6011329933655299); // false
luhnCheck(123456789); // falseMaps a number from one range to another range.
Returns num mapped between outMin-outMax from inMin-inMax.
const mapNumRange = (num, inMin, inMax, outMin, outMax) =>
((num - inMin) * (outMax - outMin)) / (inMax - inMin) + outMin;Examples
mapNumRange(5, 0, 10, 0, 100); // 50Returns the maximum value of an array, after mapping each element to a value using the provided function.
Use Array.prototype.map() to map each element to the value returned by fn, Math.max() to get the maximum value.
const maxBy = (arr, fn) => Math.max(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));Examples
maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 8
maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 8Returns the median of an array of numbers.
Find the middle of the array, use Array.prototype.sort() to sort the values.
Return the number at the midpoint if length is odd, otherwise the average of the two middle numbers.
const median = arr => {
const mid = Math.floor(arr.length / 2),
nums = [...arr].sort((a, b) => a - b);
return arr.length % 2 !== 0 ? nums[mid] : (nums[mid - 1] + nums[mid]) / 2;
};Examples
median([5, 6, 50, 1, -5]); // 5Calculates the midpoint between two pairs of (x,y) points.
Destructure the array to get x1, y1, x2 and y2, calculate the midpoint for each dimension by dividing the sum of the two endpoints by 2.
const midpoint = ([x1, y1], [x2, y2]) => [(x1 + x2) / 2, (y1 + y2) / 2];Examples
midpoint([2, 2], [4, 4]); // [3, 3]
midpoint([4, 4], [6, 6]); // [5, 5]
midpoint([1, 3], [2, 4]); // [1.5, 3.5]Returns the minimum value of an array, after mapping each element to a value using the provided function.
Use Array.prototype.map() to map each element to the value returned by fn, Math.min() to get the minimum value.
const minBy = (arr, fn) => Math.min(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));Examples
minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 2
minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 2Uses the percentile formula to calculate how many numbers in the given array are less or equal to the given value.
Use Array.prototype.reduce() to calculate how many numbers are below the value and how many are the same value and apply the percentile formula.
const percentile = (arr, val) =>
(100 * arr.reduce((acc, v) => acc + (v < val ? 1 : 0) + (v === val ? 0.5 : 0), 0)) / arr.length;Examples
percentile([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 6); // 55Returns the powerset of a given array of numbers.
Use Array.prototype.reduce() combined with Array.prototype.map() to iterate over elements and combine into an array containing all combinations.
const powerset = arr => arr.reduce((a, v) => a.concat(a.map(r => [v].concat(r))), [[]]);Examples
powerset([1, 2]); // [[], [1], [2], [2, 1]]Generates primes up to a given number, using the Sieve of Eratosthenes.
Generate an array from 2 to the given number. Use Array.prototype.filter() to filter out the values divisible by any number from 2 to the square root of the provided number.
const primes = num => {
let arr = Array.from({ length: num - 1 }).map((x, i) => i + 2),
sqroot = Math.floor(Math.sqrt(num)),
numsTillSqroot = Array.from({ length: sqroot - 1 }).map((x, i) => i + 2);
numsTillSqroot.forEach(x => (arr = arr.filter(y => y % x !== 0 || y === x)));
return arr;
};Examples
primes(10); // [2,3,5,7]Converts an angle from radians to degrees.
Use Math.PI and the radian to degree formula to convert the angle from radians to degrees.
const radsToDegrees = rad => (rad * 180.0) / Math.PI;Examples
radsToDegrees(Math.PI / 2); // 90Returns an array of n random integers in the specified range.
Use Array.from() to create an empty array of the specific length, Math.random() to generate a random number and map it to the desired range, using Math.floor() to make it an integer.
const randomIntArrayInRange = (min, max, n = 1) =>
Array.from({ length: n }, () => Math.floor(Math.random() * (max - min + 1)) + min);Examples
randomIntArrayInRange(12, 35, 10); // [ 34, 14, 27, 17, 30, 27, 20, 26, 21, 14 ]Returns a random integer in the specified range.
Use Math.random() to generate a random number and map it to the desired range, using Math.floor() to make it an integer.
const randomIntegerInRange = (min, max) => Math.floor(Math.random() * (max - min + 1)) + min;Examples
randomIntegerInRange(0, 5); // 2Returns a random number in the specified range.
Use Math.random() to generate a random value, map it to the desired range using multiplication.
const randomNumberInRange = (min, max) => Math.random() * (max - min) + min;Examples
randomNumberInRange(2, 10); // 6.0211363285087005Rounds a number to a specified amount of digits.
Use Math.round() and template literals to round the number to the specified number of digits.
Omit the second argument, decimals to round to an integer.
const round = (n, decimals = 0) => Number(`${Math.round(`${n}e${decimals}`)}e-${decimals}`);Examples
round(1.005, 2); // 1.01Hashes the input string into a whole number.
Use String.prototype.split('') and Array.prototype.reduce() to create a hash of the input string, utilizing bit shifting.
const sdbm = str => {
let arr = str.split('');
return arr.reduce(
(hashCode, currentVal) =>
(hashCode = currentVal.charCodeAt(0) + (hashCode << 6) + (hashCode << 16) - hashCode),
0
);
};Examples
sdbm('name'); // -3521204949Returns the standard deviation of an array of numbers.
Use Array.prototype.reduce() to calculate the mean, variance and the sum of the variance of the values, the variance of the values, then
determine the standard deviation.
You can omit the second argument to get the sample standard deviation or set it to true to get the population standard deviation.
const standardDeviation = (arr, usePopulation = false) => {
const mean = arr.reduce((acc, val) => acc + val, 0) / arr.length;
return Math.sqrt(
arr.reduce((acc, val) => acc.concat((val - mean) ** 2), []).reduce((acc, val) => acc + val, 0) /
(arr.length - (usePopulation ? 0 : 1))
);
};Examples
standardDeviation([10, 2, 38, 23, 38, 23, 21]); // 13.284434142114991 (sample)
standardDeviation([10, 2, 38, 23, 38, 23, 21], true); // 12.29899614287479 (population)Returns the sum of two or more numbers/arrays.
Use Array.prototype.reduce() to add each value to an accumulator, initialized with a value of 0.
const sum = (...arr) => [...arr].reduce((acc, val) => acc + val, 0);Examples
sum(1, 2, 3, 4); // 10
sum(...[1, 2, 3, 4]); // 10Returns the sum of an array, after mapping each element to a value using the provided function.
Use Array.prototype.map() to map each element to the value returned by fn, Array.prototype.reduce() to add each value to an accumulator, initialized with a value of 0.
const sumBy = (arr, fn) =>
arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => acc + val, 0);Examples
sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 20
sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 20Returns the sum of the powers of all the numbers from start to end (both inclusive).
Use Array.prototype.fill() to create an array of all the numbers in the target range, Array.prototype.map() and the exponent operator (**) to raise them to power and Array.prototype.reduce() to add them together.
Omit the second argument, power, to use a default power of 2.
Omit the third argument, start, to use a default starting value of 1.
const sumPower = (end, power = 2, start = 1) =>
Array(end + 1 - start)
.fill(0)
.map((x, i) => (i + start) ** power)
.reduce((a, b) => a + b, 0);Examples
sumPower(10); // 385
sumPower(10, 3); // 3025
sumPower(10, 3, 5); // 2925Converts a value to a safe integer.
Use Math.max() and Math.min() to find the closest safe value.
Use Math.round() to convert to an integer.
const toSafeInteger = num =>
Math.round(Math.max(Math.min(num, Number.MAX_SAFE_INTEGER), Number.MIN_SAFE_INTEGER));Examples
toSafeInteger('3.2'); // 3
toSafeInteger(Infinity); // 9007199254740991Returns the distance between two vectors.
Use Array.prototype.reduce(), Math.pow() and Math.sqrt() to calculate the Euclidean distance between two vectors.
const vectorDistance = (...coords) => {
let pointLength = Math.trunc(coords.length / 2);
let sum = coords
.slice(0, pointLength)
.reduce((acc, val, i) => acc + Math.pow(val - coords[pointLength + i], 2), 0);
return Math.sqrt(sum);
};Examples
vectorDistance(10, 0, 5, 20, 0, 10); // 11.180339887498949Decodes a string of data which has been encoded using base-64 encoding.
Create a Buffer for the given string with base-64 encoding and use Buffer.toString('binary') to return the decoded string.
const atob = str => Buffer.from(str, 'base64').toString('binary');Examples
atob('Zm9vYmFy'); // 'foobar'Creates a base-64 encoded ASCII string from a String object in which each character in the string is treated as a byte of binary data.
Create a Buffer for the given string with binary encoding and use Buffer.toString('base64') to return the encoded string.
const btoa = str => Buffer.from(str, 'binary').toString('base64');Examples
btoa('foobar'); // 'Zm9vYmFy'Add special characters to text to print in color in the console (combined with console.log()).
Use template literals and special characters to add the appropriate color code to the string output. For background colors, add a special character that resets the background color at the end of the string.
const colorize = (...args) => ({
black: `\x1b[30m${args.join(' ')}`,
red: `\x1b[31m${args.join(' ')}`,
green: `\x1b[32m${args.join(' ')}`,
yellow: `\x1b[33m${args.join(' ')}`,
blue: `\x1b[34m${args.join(' ')}`,
magenta: `\x1b[35m${args.join(' ')}`,
cyan: `\x1b[36m${args.join(' ')}`,
white: `\x1b[37m${args.join(' ')}`,
bgBlack: `\x1b[40m${args.join(' ')}\x1b[0m`,
bgRed: `\x1b[41m${args.join(' ')}\x1b[0m`,
bgGreen: `\x1b[42m${args.join(' ')}\x1b[0m`,
bgYellow: `\x1b[43m${args.join(' ')}\x1b[0m`,
bgBlue: `\x1b[44m${args.join(' ')}\x1b[0m`,
bgMagenta: `\x1b[45m${args.join(' ')}\x1b[0m`,
bgCyan: `\x1b[46m${args.join(' ')}\x1b[0m`,
bgWhite: `\x1b[47m${args.join(' ')}\x1b[0m`
});Examples
console.log(colorize('foo').red); // 'foo' (red letters)
console.log(colorize('foo', 'bar').bgBlue); // 'foo bar' (blue background)
console.log(colorize(colorize('foo').yellow, colorize('foo').green).bgWhite); // 'foo bar' (first word in yellow letters, second word in green letters, white background for both)Creates a directory, if it does not exist.
Use fs.existsSync() to check if the directory exists, fs.mkdirSync() to create it.
const fs = require('fs');
const createDirIfNotExists = dir => (!fs.existsSync(dir) ? fs.mkdirSync(dir) : undefined);Examples
createDirIfNotExists('test'); // creates the directory 'test', if it doesn't existCheck if the current process's arguments contain the specified flags.
Use Array.prototype.every() and Array.prototype.includes() to check if process.argv contains all the specified flags.
Use a regular expression to test if the specified flags are prefixed with - or -- and prefix them accordingly.
const hasFlags = (...flags) =>
flags.every(flag => process.argv.includes(/^-{1,2}/.test(flag) ? flag : '--' + flag));Examples
// node myScript.js -s --test --cool=true
hasFlags('-s'); // true
hasFlags('--test', 'cool=true', '-s'); // true
hasFlags('special'); // falseCreates a hash for a value using the SHA-256 algorithm. Returns a promise.
Use crypto API to create a hash for the given value, setTimeout to prevent blocking on a long operation, and a Promise to give it a familiar interface.
const crypto = require('crypto');
const hashNode = val =>
new Promise(resolve =>
setTimeout(
() =>
resolve(
crypto
.createHash('sha256')
.update(val)
.digest('hex')
),
0
)
);Examples
hashNode(JSON.stringify({ a: 'a', b: [1, 2, 3, 4], foo: { c: 'bar' } })).then(console.log); // '04aa106279f5977f59f9067fa9712afc4aedc6f5862a8defc34552d8c7206393'Checks if the given argument is a duplex (readable and writable) stream.
Check if the value is different from null, use typeof to check if a value is of type object and the pipe property is of type function.
Additionally check if the typeof the _read, _write and _readableState, _writableState properties are function and object respectively.
const isDuplexStream = val =>
val !== null &&
typeof val === 'object' &&
typeof val.pipe === 'function' &&
typeof val._read === 'function' &&
typeof val._readableState === 'object' &&
typeof val._write === 'function' &&
typeof val._writableState === 'object';Examples
const Stream = require('stream');
isDuplexStream(new Stream.Duplex()); // trueChecks if the given argument is a readable stream.
Check if the value is different from null, use typeof to check if the value is of type object and the pipe property is of type function.
Additionally check if the typeof the _read and _readableState properties are function and object respectively.
const isReadableStream = val =>
val !== null &&
typeof val === 'object' &&
typeof val.pipe === 'function' &&
typeof val._read === 'function' &&
typeof val._readableState === 'object';Examples
const fs = require('fs');
isReadableStream(fs.createReadStream('test.txt')); // trueChecks if the given argument is a stream.
Check if the value is different from null, use typeof to check if the value is of type object and the pipe property is of type function.
const isStream = val => val !== null && typeof val === 'object' && typeof val.pipe === 'function';Examples
const fs = require('fs');
isStream(fs.createReadStream('test.txt')); // trueChecks if the current environment is Travis CI.
Checks if the current environment has the TRAVIS and CI environment variables (reference).
const isTravisCI = () => 'TRAVIS' in process.env && 'CI' in process.env;Examples
isTravisCI(); // true (if code is running on Travis CI)Checks if the given argument is a writable stream.
Check if the value is different from null, use typeof to check if the value is of type object and the pipe property is of type function.
Additionally check if the typeof the _write and _writableState properties are function and object respectively.
const isWritableStream = val =>
val !== null &&
typeof val === 'object' &&
typeof val.pipe === 'function' &&
typeof val._write === 'function' &&
typeof val._writableState === 'object';Examples
const fs = require('fs');
isWritableStream(fs.createWriteStream('test.txt')); // trueWrites a JSON object to a file.
Use fs.writeFile(), template literals and JSON.stringify() to write a json object to a .json file.
const fs = require('fs');
const JSONToFile = (obj, filename) =>
fs.writeFile(`${filename}.json`, JSON.stringify(obj, null, 2));Examples
JSONToFile({ test: 'is passed' }, 'testJsonFile'); // writes the object to 'testJsonFile.json'Returns an array of lines from the specified file.
Use readFileSync function in fs node package to create a Buffer from a file.
convert buffer to string using toString(encoding) function.
creating an array from contents of file by spliting file content line by line (each \n).
const fs = require('fs');
const readFileLines = filename =>
fs
.readFileSync(filename)
.toString('UTF8')
.split('\n');Examples
/*
contents of test.txt :
line1
line2
line3
___________________________
*/
let arr = readFileLines('test.txt');
console.log(arr); // ['line1', 'line2', 'line3']Converts a tilde path to an absolute path.
Use String.prototype.replace() with a regular expression and OS.homedir() to replace the ~ in the start of the path with the home directory.
const untildify = str => str.replace(/^~($|\/|\\)/, `${require('os').homedir()}$1`);Examples
untildify('~/node'); // '/Users/aUser/node'Generates a UUID in Node.JS.
Use crypto API to generate a UUID, compliant with RFC4122 version 4.
const crypto = require('crypto');
const UUIDGeneratorNode = () =>
([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
(c ^ (crypto.randomBytes(1)[0] & (15 >> (c / 4)))).toString(16)
);Examples
UUIDGeneratorNode(); // '79c7c136-60ee-40a2-beb2-856f1feabefc'Binds methods of an object to the object itself, overwriting the existing method.
Use Array.prototype.forEach() to return a function that uses Function.prototype.apply() to apply the given context (obj) to fn for each function specified.
const bindAll = (obj, ...fns) =>
fns.forEach(
fn => (
(f = obj[fn]),
(obj[fn] = function() {
return f.apply(obj);
})
)
);Examples
var view = {
label: 'docs',
click: function() {
console.log('clicked ' + this.label);
}
};
bindAll(view, 'click');
jQuery(element).on('click', view.click); // Logs 'clicked docs' when clicked.Creates a deep clone of an object.
Use recursion.
Check if the passed object is null and, if so, return null.
Use Object.assign() and an empty object ({}) to create a shallow clone of the original.
Use Object.keys() and Array.prototype.forEach() to determine which key-value pairs need to be deep cloned.
const deepClone = obj => {
if (obj === null) return null;
let clone = Object.assign({}, obj);
Object.keys(clone).forEach(
key => (clone[key] = typeof obj[key] === 'object' ? deepClone(obj[key]) : obj[key])
);
return Array.isArray(obj) && obj.length
? (clone.length = obj.length) && Array.from(clone)
: Array.isArray(obj)
? Array.from(obj)
: clone;
};Examples
const a = { foo: 'bar', obj: { a: 1, b: 2 } };
const b = deepClone(a); // a !== b, a.obj !== b.objDeep freezes an object.
Calls Object.freeze(obj) recursively on all unfrozen properties of passed object that are instanceof object.
const deepFreeze = obj =>
Object.keys(obj).forEach(prop =>
!(obj[prop] instanceof Object) || Object.isFrozen(obj[prop]) ? null : deepFreeze(obj[prop])
) || Object.freeze(obj);Examples
'use strict';
const o = deepFreeze([1, [2, 3]]);
o[0] = 3; // not allowed
o[1][0] = 4; // not allowed as wellReturns the target value in a nested JSON object, based on the keys array.
Compare the keys you want in the nested JSON object as an Array.
Use Array.prototype.reduce() to get value from nested JSON object one by one.
If the key exists in object, return target value, otherwise, return null.
const deepGet = (obj, keys) => keys.reduce((xs, x) => (xs && xs[x] ? xs[x] : null), obj);Examples
let index = 2;
const data = {
foo: {
foz: [1, 2, 3],
bar: {
baz: ['a', 'b', 'c']
}
}
};
deepGet(data, ['foo', 'foz', index]); // get 3
deepGet(data, ['foo', 'bar', 'baz', 8, 'foz']); // nullDeep maps an object's keys.
Creates an object with the same values as the provided object and keys generated by running the provided function for each key.
Use Object.keys(obj) to iterate over the object's keys.
Use Array.prototype.reduce() to create a new object with the same values and mapped keys using fn.
const deepMapKeys = (obj, f) =>
Array.isArray(obj)
? obj.map(val => deepMapKeys(val, f))
: typeof obj === 'object'
? Object.keys(obj).reduce((acc, current) => {
const val = obj[current];
acc[f(current)] =
val !== null && typeof val === 'object' ? deepMapKeys(val, f) : (acc[f(current)] = val);
return acc;
}, {})
: obj;Examples
const obj = {
foo: '1',
nested: {
child: {
withArray: [
{
grandChild: ['hello']
}
]
}
}
};
const upperKeysObj = deepMapKeys(obj, key => key.toUpperCase());
/*
{
"FOO":"1",
"NESTED":{
"CHILD":{
"WITHARRAY":[
{
"GRANDCHILD":[ 'hello' ]
}
]
}
}
}
*/Assigns default values for all properties in an object that are undefined.
Use Object.assign() to create a new empty object and copy the original one to maintain key order, use Array.prototype.reverse() and the spread operator ... to combine the default values from left to right, finally use obj again to overwrite properties that originally had a value.
const defaults = (obj, ...defs) => Object.assign({}, obj, ...defs.reverse(), obj);Examples
defaults({ a: 1 }, { b: 2 }, { b: 6 }, { a: 3 }); // { a: 1, b: 2 }Returns the target value in a nested JSON object, based on the given key.
Use the in operator to check if target exists in obj.
If found, return the value of obj[target], otherwise use Object.values(obj) and Array.prototype.reduce() to recursively call dig on each nested object until the first matching key/value pair is found.
const dig = (obj, target) =>
target in obj
? obj[target]
: Object.values(obj).reduce((acc, val) => {
if (acc !== undefined) return acc;
if (typeof val === 'object') return dig(val, target);
}, undefined);Examples
const data = {
level1: {
level2: {
level3: 'some data'
}
}
};
dig(data, 'level3'); // 'some data'
dig(data, 'level4'); // undefinedPerforms a deep comparison between two values to determine if they are equivalent.
Check if the two values are identical, if they are both Date objects with the same time, using Date.getTime() or if they are both non-object values with an equivalent value (strict comparison).
Check if only one value is null or undefined or if their prototypes differ.
If none of the above conditions are met, use Object.keys() to check if both values have the same number of keys, then use Array.prototype.every() to check if every key in the first value exists in the second one and if they are equivalent by calling this method recursively.
const equals = (a, b) => {
if (a === b) return true;
if (a instanceof Date && b instanceof Date) return a.getTime() === b.getTime();
if (!a || !b || (typeof a !== 'object' && typeof b !== 'object')) return a === b;
if (a.prototype !== b.prototype) return false;
let keys = Object.keys(a);
if (keys.length !== Object.keys(b).length) return false;
return keys.every(k => equals(a[k], b[k]));
};Examples
equals({ a: [2, { e: 3 }], b: [4], c: 'foo' }, { a: [2, { e: 3 }], b: [4], c: 'foo' }); // trueReturns the first key that satisfies the provided testing function. Otherwise undefined is returned.
Use Object.keys(obj) to get all the properties of the object, Array.prototype.find() to test the provided function for each key-value pair. The callback receives three arguments - the value, the key and the object.
const findKey = (obj, fn) => Object.keys(obj).find(key => fn(obj[key], key, obj));Examples
findKey(
{
barney: { age: 36, active: true },
fred: { age: 40, active: false },
pebbles: { age: 1, active: true }
},
o => o['active']
); // 'barney'Returns the last key that satisfies the provided testing function.
Otherwise undefined is returned.
Use Object.keys(obj) to get all the properties of the object, Array.prototype.reverse() to reverse their order and Array.prototype.find() to test the provided function for each key-value pair.
The callback receives three arguments - the value, the key and the object.
const findLastKey = (obj, fn) =>
Object.keys(obj)
.reverse()
.find(key => fn(obj[key], key, obj));Examples
findLastKey(
{
barney: { age: 36, active: true },
fred: { age: 40, active: false },
pebbles: { age: 1, active: true }
},
o => o['active']
); // 'pebbles'Flatten an object with the paths for keys.
Use recursion.
Use Object.keys(obj) combined with Array.prototype.reduce() to convert every leaf node to a flattened path node.
If the value of a key is an object, the function calls itself with the appropriate prefix to create the path using Object.assign().
Otherwise, it adds the appropriate prefixed key-value pair to the accumulator object.
You should always omit the second argument, prefix, unless you want every key to have a prefix.
const flattenObject = (obj, prefix = '') =>
Object.keys(obj).reduce((acc, k) => {
const pre = prefix.length ? prefix + '.' : '';
if (typeof obj[k] === 'object') Object.assign(acc, flattenObject(obj[k], pre + k));
else acc[pre + k] = obj[k];
return acc;
}, {});Examples
flattenObject({ a: { b: { c: 1 } }, d: 1 }); // { 'a.b.c': 1, d: 1 }Iterates over all own properties of an object, running a callback for each one.
Use Object.keys(obj) to get all the properties of the object, Array.prototype.forEach() to run the provided function for each key-value pair. The callback receives three arguments - the value, the key and the object.
const forOwn = (obj, fn) => Object.keys(obj).forEach(key => fn(obj[key], key, obj));Examples
forOwn({ foo: 'bar', a: 1 }, v => console.log(v)); // 'bar', 1Iterates over all own properties of an object in reverse, running a callback for each one.
Use Object.keys(obj) to get all the properties of the object, Array.prototype.reverse() to reverse their order and Array.prototype.forEach() to run the provided function for each key-value pair. The callback receives three arguments - the value, the key and the object.
const forOwnRight = (obj, fn) =>
Object.keys(obj)
.reverse()
.forEach(key => fn(obj[key], key, obj));Examples
forOwnRight({ foo: 'bar', a: 1 }, v => console.log(v)); // 1, 'bar'Returns an array of function property names from own (and optionally inherited) enumerable properties of an object.
Use Object.keys(obj) to iterate over the object's own properties.
If inherited is true, use Object.get.PrototypeOf(obj) to also get the object's inherited properties.
Use Array.prototype.filter() to keep only those properties that are functions.
Omit the second argument, inherited, to not include inherited properties by default.
const functions = (obj, inherited = false) =>
(inherited
? [...Object.keys(obj), ...Object.keys(Object.getPrototypeOf(obj))]
: Object.keys(obj)
).filter(key => typeof obj[key] === 'function');Examples
function Foo() {
this.a = () => 1;
this.b = () => 2;
}
Foo.prototype.c = () => 3;
functions(new Foo()); // ['a', 'b']
functions(new Foo(), true); // ['a', 'b', 'c']Retrieve a set of properties indicated by the given selectors from an object.
Use Array.prototype.map() for each selector, String.prototype.replace() to replace square brackets with dots, String.prototype.split('.') to split each selector, Array.prototype.filter() to remove empty values and Array.prototype.reduce() to get the value indicated by it.
const get = (from, ...selectors) =>
[...selectors].map(s =>
s
.replace(/\[([^\[\]]*)\]/g, '.$1.')
.split('.')
.filter(t => t !== '')
.reduce((prev, cur) => prev && prev[cur], from)
);Examples
const obj = { selector: { to: { val: 'val to select' } }, target: [1, 2, { a: 'test' }] };
get(obj, 'selector.to.val', 'target[0]', 'target[2].a'); // ['val to select', 1, 'test']Returns true if the target value exists in a JSON object, false otherwise.
Check if keys is non-empty and use Array.prototype.every() to sequentially check its keys to internal depth of the object, obj.
Use Object.prototype.hasOwnProperty() to check if obj does not have the current key or is not an object, stop propagation and return false.
Otherwise assign the key's value to obj to use on the next iteration.
Return false beforehand if given key list is empty.
const hasKey = (obj, keys) => {
return (
keys.length > 0 &&
keys.every(key => {
if (typeof obj !== 'object' || !obj.hasOwnProperty(key)) return false;
obj = obj[key];
return true;
})
);
};Examples
let obj = {
a: 1,
b: { c: 4 },
'b.d': 5
};
hasKey(obj, ['a']); // true
hasKey(obj, ['b']); // true
hasKey(obj, ['b', 'c']); // true
hasKey(obj, ['b.d']); // true
hasKey(obj, ['d']); // false
hasKey(obj, ['c']); // false
hasKey(obj, ['b', 'f']); // falseInverts the key-value pairs of an object, without mutating it. The corresponding inverted value of each inverted key is an array of keys responsible for generating the inverted value. If a function is supplied, it is applied to each inverted key.
Use Object.keys() and Array.prototype.reduce() to invert the key-value pairs of an object and apply the function provided (if any).
Omit the second argument, fn, to get the inverted keys without applying a function to them.
const invertKeyValues = (obj, fn) =>
Object.keys(obj).reduce((acc, key) => {
const val = fn ? fn(obj[key]) : obj[key];
acc[val] = acc[val] || [];
acc[val].push(key);
return acc;
}, {});Examples
invertKeyValues({ a: 1, b: 2, c: 1 }); // { 1: [ 'a', 'c' ], 2: [ 'b' ] }
invertKeyValues({ a: 1, b: 2, c: 1 }, value => 'group' + value); // { group1: [ 'a', 'c' ], group2: [ 'b' ] }Creates a new object from the specified object, where all the keys are in lowercase.
Use Object.keys() and Array.prototype.reduce() to create a new object from the specified object.
Convert each key in the original object to lowercase, using String.toLowerCase().
const lowercaseKeys = obj =>
Object.keys(obj).reduce((acc, key) => {
acc[key.toLowerCase()] = obj[key];
return acc;
}, {});Examples
const myObj = { Name: 'Adam', sUrnAME: 'Smith' };
const myObjLower = lowercaseKeys(myObj); // {name: 'Adam', surname: 'Smith'};Creates an object with keys generated by running the provided function for each key and the same values as the provided object.
Use Object.keys(obj) to iterate over the object's keys.
Use Array.prototype.reduce() to create a new object with the same values and mapped keys using fn.
const mapKeys = (obj, fn) =>
Object.keys(obj).reduce((acc, k) => {
acc[fn(obj[k], k, obj)] = obj[k];
return acc;
}, {});Examples
mapKeys({ a: 1, b: 2 }, (val, key) => key + val); // { a1: 1, b2: 2 }Creates an object with the same keys as the provided object and values generated by running the provided function for each value.
Use Object.keys(obj) to iterate over the object's keys.
Use Array.prototype.reduce() to create a new object with the same keys and mapped values using fn.
const mapValues = (obj, fn) =>
Object.keys(obj).reduce((acc, k) => {
acc[k] = fn(obj[k], k, obj);
return acc;
}, {});Examples
const users = {
fred: { user: 'fred', age: 40 },
pebbles: { user: 'pebbles', age: 1 }
};
mapValues(users, u => u.age); // { fred: 40, pebbles: 1 }Compares two objects to determine if the first one contains equivalent property values to the second one.
Use Object.keys(source) to get all the keys of the second object, then Array.prototype.every(), Object.hasOwnProperty() and strict comparison to determine if all keys exist in the first object and have the same values.
const matches = (obj, source) =>
Object.keys(source).every(key => obj.hasOwnProperty(key) && obj[key] === source[key]);Examples
matches({ age: 25, hair: 'long', beard: true }, { hair: 'long', beard: true }); // true
matches({ hair: 'long', beard: true }, { age: 25, hair: 'long', beard: true }); // falseCompares two objects to determine if the first one contains equivalent property values to the second one, based on a provided function.
Use Object.keys(source) to get all the keys of the second object, then Array.prototype.every(), Object.hasOwnProperty() and the provided function to determine if all keys exist in the first object and have equivalent values.
If no function is provided, the values will be compared using the equality operator.
const matchesWith = (obj, source, fn) =>
Object.keys(source).every(key =>
obj.hasOwnProperty(key) && fn
? fn(obj[key], source[key], key, obj, source)
: obj[key] == source[key]
);Examples
const isGreeting = val => /^h(?:i|ello)$/.test(val);
matchesWith(
{ greeting: 'hello' },
{ greeting: 'hi' },
(oV, sV) => isGreeting(oV) && isGreeting(sV)
); // trueCreates a new object from the combination of two or more objects.
Use Array.prototype.reduce() combined with Object.keys(obj) to iterate over all objects and keys.
Use hasOwnProperty() and Array.prototype.concat() to append values for keys existing in multiple objects.
const merge = (...objs) =>
[...objs].reduce(
(acc, obj) =>
Object.keys(obj).reduce((a, k) => {
acc[k] = acc.hasOwnProperty(k) ? [].concat(acc[k]).concat(obj[k]) : obj[k];
return acc;
}, {}),
{}
);Examples
const object = {
a: [{ x: 2 }, { y: 4 }],
b: 1
};
const other = {
a: { z: 3 },
b: [2, 3],
c: 'foo'
};
merge(object, other); // { a: [ { x: 2 }, { y: 4 }, { z: 3 } ], b: [ 1, 2, 3 ], c: 'foo' }Given a flat array of objects linked to one another, it will nest them recursively. Useful for nesting comments, such as the ones on reddit.com.
Use recursion.
Use Array.prototype.filter() to filter the items where the id matches the link, then Array.prototype.map() to map each one to a new object that has a children property which recursively nests the items based on which ones are children of the current item.
Omit the second argument, id, to default to null which indicates the object is not linked to another one (i.e. it is a top level object).
Omit the third argument, link, to use 'parent_id' as the default property which links the object to another one by its id.
const nest = (items, id = null, link = 'parent_id') =>
items
.filter(item => item[link] === id)
.map(item => ({ ...item, children: nest(items, item.id) }));Examples
// One top level comment
const comments = [
{ id: 1, parent_id: null },
{ id: 2, parent_id: 1 },
{ id: 3, parent_id: 1 },
{ id: 4, parent_id: 2 },
{ id: 5, parent_id: 4 }
];
const nestedComments = nest(comments); // [{ id: 1, parent_id: null, children: [...] }]Creates an object from the given key-value pairs.
Use Array.prototype.reduce() to create and combine key-value pairs.
const objectFromPairs = arr => arr.reduce((a, [key, val]) => ((a[key] = val), a), {});Examples
objectFromPairs([['a', 1], ['b', 2]]); // {a: 1, b: 2}Creates an array of key-value pair arrays from an object.
Use Object.keys() and Array.prototype.map() to iterate over the object's keys and produce an array with key-value pairs.
const objectToPairs = obj => Object.keys(obj).map(k => [k, obj[k]]);Examples
objectToPairs({ a: 1, b: 2 }); // [ ['a', 1], ['b', 2] ]Omits the key-value pairs corresponding to the given keys from an object.
Use Object.keys(obj), Array.prototype.filter() and Array.prototype.includes() to remove the provided keys.
Use Array.prototype.reduce() to convert the filtered keys back to an object with the corresponding key-value pairs.
const omit = (obj, arr) =>
Object.keys(obj)
.filter(k => !arr.includes(k))
.reduce((acc, key) => ((acc[key] = obj[key]), acc), {});Examples
omit({ a: 1, b: '2', c: 3 }, ['b']); // { 'a': 1, 'c': 3 }Creates an object composed of the properties the given function returns falsy for. The function is invoked with two arguments: (value, key).
Use Object.keys(obj) and Array.prototype.filter()to remove the keys for which fn returns a truthy value.
Use Array.prototype.reduce() to convert the filtered keys back to an object with the corresponding key-value pairs.
const omitBy = (obj, fn) =>
Object.keys(obj)
.filter(k => !fn(obj[k], k))
.reduce((acc, key) => ((acc[key] = obj[key]), acc), {});Examples
omitBy({ a: 1, b: '2', c: 3 }, x => typeof x === 'number'); // { b: '2' }Returns a sorted array of objects ordered by properties and orders.
Uses Array.prototype.sort(), Array.prototype.reduce() on the props array with a default value of 0, use array destructuring to swap the properties position depending on the order passed.
If no orders array is passed it sort by 'asc' by default.
const orderBy = (arr, props, orders) =>
[...arr].sort((a, b) =>
props.reduce((acc, prop, i) => {
if (acc === 0) {
const [p1, p2] = orders && orders[i] === 'desc' ? [b[prop], a[prop]] : [a[prop], b[prop]];
acc = p1 > p2 ? 1 : p1 < p2 ? -1 : 0;
}
return acc;
}, 0)
);Examples
const users = [{ name: 'fred', age: 48 }, { name: 'barney', age: 36 }, { name: 'fred', age: 40 }];
orderBy(users, ['name', 'age'], ['asc', 'desc']); // [{name: 'barney', age: 36}, {name: 'fred', age: 48}, {name: 'fred', age: 40}]
orderBy(users, ['name', 'age']); // [{name: 'barney', age: 36}, {name: 'fred', age: 40}, {name: 'fred', age: 48}]Picks the key-value pairs corresponding to the given keys from an object.
Use Array.prototype.reduce() to convert the filtered/picked keys back to an object with the corresponding key-value pairs if the key exists in the object.
const pick = (obj, arr) =>
arr.reduce((acc, curr) => (curr in obj && (acc[curr] = obj[curr]), acc), {});Examples
pick({ a: 1, b: '2', c: 3 }, ['a', 'c']); // { 'a': 1, 'c': 3 }Creates an object composed of the properties the given function returns truthy for. The function is invoked with two arguments: (value, key).
Use Object.keys(obj) and Array.prototype.filter()to remove the keys for which fn returns a falsy value.
Use Array.prototype.reduce() to convert the filtered keys back to an object with the corresponding key-value pairs.
const pickBy = (obj, fn) =>
Object.keys(obj)
.filter(k => fn(obj[k], k))
.reduce((acc, key) => ((acc[key] = obj[key]), acc), {});Examples
pickBy({ a: 1, b: '2', c: 3 }, x => typeof x === 'number'); // { 'a': 1, 'c': 3 }Replaces the names of multiple object keys with the values provided.
Use Object.keys() in combination with Array.prototype.reduce() and the spread operator (...) to get the object's keys and rename them according to keysMap.
const renameKeys = (keysMap, obj) =>
Object.keys(obj).reduce(
(acc, key) => ({
...acc,
...{ [keysMap[key] || key]: obj[key] }
}),
{}
);Examples
const obj = { name: 'Bobo', job: 'Front-End Master', shoeSize: 100 };
renameKeys({ name: 'firstName', job: 'passion' }, obj); // { firstName: 'Bobo', passion: 'Front-End Master', shoeSize: 100 }Creates a shallow clone of an object.
Use Object.assign() and an empty object ({}) to create a shallow clone of the original.
const shallowClone = obj => Object.assign({}, obj);Examples
const a = { x: true, y: 1 };
const b = shallowClone(a); // a !== bGet size of arrays, objects or strings.
Get type of val (array, object or string).
Use length property for arrays.
Use length or size value if available or number of keys for objects.
Use size of a Blob object created from val for strings.
Split strings into array of characters with split('') and return its length.
const size = val =>
Array.isArray(val)
? val.length
: val && typeof val === 'object'
? val.size || val.length || Object.keys(val).length
: typeof val === 'string'
? new Blob([val]).size
: 0;Examples
size([1, 2, 3, 4, 5]); // 5
size('size'); // 4
size({ one: 1, two: 2, three: 3 }); // 3Applies a function against an accumulator and each key in the object (from left to right).
Use Object.keys(obj) to iterate over each key in the object, Array.prototype.reduce() to call the apply the specified function against the given accumulator.
const transform = (obj, fn, acc) => Object.keys(obj).reduce((a, k) => fn(a, obj[k], k, obj), acc);Examples
transform(
{ a: 1, b: 2, c: 1 },
(r, v, k) => {
(r[v] || (r[v] = [])).push(k);
return r;
},
{}
); // { '1': ['a', 'c'], '2': ['b'] }Checks if the predicate (second argument) is truthy on all elements of a collection (first argument).
Use Array.prototype.every() to check if each passed object has the specified property and if it returns a truthy value.
const truthCheckCollection = (collection, pre) => collection.every(obj => obj[pre]);Examples
truthCheckCollection([{ user: 'Tinky-Winky', sex: 'male' }, { user: 'Dipsy', sex: 'male' }], 'sex'); // trueUnflatten an object with the paths for keys.
Use Object.keys(obj) combined with Array.prototype.reduce() to convert flattened path node to a leaf node.
If the value of a key contains a dot delimiter (.), use Array.prototype.split('.'), string transformations and JSON.parse() to create an object, then Object.assign() to create the leaf node.
Otherwise, add the appropriate key-value pair to the accumulator object.
const unflattenObject = obj =>
Object.keys(obj).reduce((acc, k) => {
if (k.indexOf('.') !== -1) {
const keys = k.split('.');
Object.assign(
acc,
JSON.parse(
'{' +
keys.map((v, i) => (i !== keys.length - 1 ? `"${v}":{` : `"${v}":`)).join('') +
obj[k] +
'}'.repeat(keys.length)
)
);
} else acc[k] = obj[k];
return acc;
}, {});Examples
unflattenObject({ 'a.b.c': 1, d: 1 }); // { a: { b: { c: 1 } }, d: 1 }Returns the length of a string in bytes.
Convert a given string to a Blob Object and find its size.
const byteSize = str => new Blob([str]).size;Examples
byteSize('😀'); // 4
byteSize('Hello World'); // 11Capitalizes the first letter of a string.
Use array destructuring and String.prototype.toUpperCase() to capitalize first letter, ...rest to get array of characters after first letter and then Array.prototype.join('') to make it a string again.
Omit the lowerRest parameter to keep the rest of the string intact, or set it to true to convert to lowercase.
const capitalize = ([first, ...rest], lowerRest = false) =>
first.toUpperCase() + (lowerRest ? rest.join('').toLowerCase() : rest.join(''));Examples
capitalize('fooBar'); // 'FooBar'
capitalize('fooBar', true); // 'Foobar'Capitalizes the first letter of every word in a string.
Use String.prototype.replace() to match the first character of each word and String.prototype.toUpperCase() to capitalize it.
const capitalizeEveryWord = str => str.replace(/\b[a-z]/g, char => char.toUpperCase());Examples
capitalizeEveryWord('hello world!'); // 'Hello World!'Returns a string with whitespaces compacted.
Use String.prototype.replace() with a regular expression to replace all occurrences of 2 or more whitespace characters with a single space.
const compactWhitespace = str => str.replace(/\s{2,}/g, ' ');Examples
compactWhitespace('Lorem Ipsum'); // 'Lorem Ipsum'
compactWhitespace('Lorem \n Ipsum'); // 'Lorem Ipsum'Converts a comma-separated values (CSV) string to a 2D array.
Use Array.prototype.slice() and Array.prototype.indexOf('\n') to remove the first row (title row) if omitFirstRow is true.
Use String.prototype.split('\n') to create a string for each row, then String.prototype.split(delimiter) to separate the values in each row.
Omit the second argument, delimiter, to use a default delimiter of ,.
Omit the third argument, omitFirstRow, to include the first row (title row) of the CSV string.
const CSVToArray = (data, delimiter = ',', omitFirstRow = false) =>
data
.slice(omitFirstRow ? data.indexOf('\n') + 1 : 0)
.split('\n')
.map(v => v.split(delimiter));Examples
CSVToArray('a,b\nc,d'); // [['a','b'],['c','d']];
CSVToArray('a;b\nc;d', ';'); // [['a','b'],['c','d']];
CSVToArray('col1,col2\na,b\nc,d', ',', true); // [['a','b'],['c','d']];Converts a comma-separated values (CSV) string to a 2D array of objects. The first row of the string is used as the title row.
Use Array.prototype.slice() and Array.prototype.indexOf('\n') and String.prototype.split(delimiter) to separate the first row (title row) into values.
Use String.prototype.split('\n') to create a string for each row, then Array.prototype.map() and String.prototype.split(delimiter) to separate the values in each row.
Use Array.prototype.reduce() to create an object for each row's values, with the keys parsed from the title row.
Omit the second argument, delimiter, to use a default delimiter of ,.
const CSVToJSON = (data, delimiter = ',') => {
const titles = data.slice(0, data.indexOf('\n')).split(delimiter);
return data
.slice(data.indexOf('\n') + 1)
.split('\n')
.map(v => {
const values = v.split(delimiter);
return titles.reduce((obj, title, index) => ((obj[title] = values[index]), obj), {});
});
};Examples
CSVToJSON('col1,col2\na,b\nc,d'); // [{'col1': 'a', 'col2': 'b'}, {'col1': 'c', 'col2': 'd'}];
CSVToJSON('col1;col2\na;b\nc;d', ';'); // [{'col1': 'a', 'col2': 'b'}, {'col1': 'c', 'col2': 'd'}];Decapitalizes the first letter of a string.
Use array destructuring and String.toLowerCase() to decapitalize first letter, ...rest to get array of characters after first letter and then Array.prototype.join('') to make it a string again.
Omit the upperRest parameter to keep the rest of the string intact, or set it to true to convert to uppercase.
const decapitalize = ([first, ...rest], upperRest = false) =>
first.toLowerCase() + (upperRest ? rest.join('').toUpperCase() : rest.join(''));Examples
decapitalize('FooBar'); // 'fooBar'
decapitalize('FooBar', true); // 'fOOBAR'Escapes a string for use in HTML.
Use String.prototype.replace() with a regexp that matches the characters that need to be escaped, using a callback function to replace each character instance with its associated escaped character using a dictionary (object).
const escapeHTML = str =>
str.replace(
/[&<>'"]/g,
tag =>
({
'&': '&',
'<': '<',
'>': '>',
"'": ''',
'"': '"'
}[tag] || tag)
);Examples
escapeHTML('<a href="#">Me & you</a>'); // '<a href="#">Me & you</a>'Escapes a string to use in a regular expression.
Use String.prototype.replace() to escape special characters.
const escapeRegExp = str => str.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');Examples
escapeRegExp('(test)'); // \\(test\\)Converts a string from camelcase.
Use String.prototype.replace() to remove underscores, hyphens, and spaces and convert words to camelcase.
Omit the second argument to use a default separator of _.
const fromCamelCase = (str, separator = '_') =>
str
.replace(/([a-z\d])([A-Z])/g, '$1' + separator + '$2')
.replace(/([A-Z]+)([A-Z][a-z\d]+)/g, '$1' + separator + '$2')
.toLowerCase();Examples
fromCamelCase('someDatabaseFieldName', ' '); // 'some database field name'
fromCamelCase('someLabelThatNeedsToBeCamelized', '-'); // 'some-label-that-needs-to-be-camelized'
fromCamelCase('someJavascriptProperty', '_'); // 'some_javascript_property'Indents each line in the provided string.
Use String.replace and a regular expression to add the character specified by indent count times at the start of each line.
Omit the third parameter, indent, to use a default indentation character of ' '.
const indentString = (str, count, indent = ' ') => str.replace(/^/gm, indent.repeat(count));Examples
indentString('Lorem\nIpsum', 2); // ' Lorem\n Ipsum'
indentString('Lorem\nIpsum', 2, '_'); // '__Lorem\n__Ipsum'Returns true if the given string is an absolute URL, false otherwise.
Use a regular expression to test if the string is an absolute URL.
const isAbsoluteURL = str => /^[a-z][a-z0-9+.-]*:/.test(str);Examples
isAbsoluteURL('https://google.com'); // true
isAbsoluteURL('ftp://www.myserver.net'); // true
isAbsoluteURL('/foo/bar'); // falseChecks if a string is an anagram of another string (case-insensitive, ignores spaces, punctuation and special characters).
Use String.toLowerCase(), String.prototype.replace() with an appropriate regular expression to remove unnecessary characters, String.prototype.split(''), Array.prototype.sort() and Array.prototype.join('') on both strings to normalize them, then check if their normalized forms are equal.
const isAnagram = (str1, str2) => {
const normalize = str =>
str
.toLowerCase()
.replace(/[^a-z0-9]/gi, '')
.split('')
.sort()
.join('');
return normalize(str1) === normalize(str2);
};Examples
isAnagram('iceman', 'cinema'); // trueChecks if a string is lower case.
Convert the given string to lower case, using String.toLowerCase() and compare it to the original.
const isLowerCase = str => str === str.toLowerCase();Examples
isLowerCase('abc'); // true
isLowerCase('a3@$'); // true
isLowerCase('Ab4'); // falseChecks if a string is upper case.
Convert the given string to upper case, using String.prototype.toUpperCase() and compare it to the original.
const isUpperCase = str => str === str.toUpperCase();Examples
isUpperCase('ABC'); // true
isUpperCase('A3@$'); // true
isUpperCase('aB4'); // falseCreates a new string with the results of calling a provided function on every character in the calling string.
Use String.prototype.split('') and Array.prototype.map() to call the provided function, fn, for each character in str.
Use Array.prototype.join('') to recombine the array of characters into a string.
The callback function, fn, takes three arguments (the current character, the index of the current character and the string mapString was called upon).
const mapString = (str, fn) =>
str
.split('')
.map((c, i) => fn(c, i, str))
.join('');Examples
mapString('lorem ipsum', c => c.toUpperCase()); // 'LOREM IPSUM'Replaces all but the last num of characters with the specified mask character.
Use String.prototype.slice() to grab the portion of the characters that will remain unmasked and use String.padStart() to fill the beginning of the string with the mask character up to the original length.
Omit the second argument, num, to keep a default of 4 characters unmasked. If num is negative, the unmasked characters will be at the start of the string.
Omit the third argument, mask, to use a default character of '*' for the mask.
const mask = (cc, num = 4, mask = '*') => `${cc}`.slice(-num).padStart(`${cc}`.length, mask);Examples
mask(1234567890); // '******7890'
mask(1234567890, 3); // '*******890'
mask(1234567890, -4, '$'); // '$$$$567890'Pads a string on both sides with the specified character, if it's shorter than the specified length.
Use String.padStart() and String.padEnd() to pad both sides of the given string.
Omit the third argument, char, to use the whitespace character as the default padding character.
const pad = (str, length, char = ' ') =>
str.padStart((str.length + length) / 2, char).padEnd(length, char);Examples
pad('cat', 8); // ' cat '
pad(String(42), 6, '0'); // '004200'
pad('foobar', 3); // 'foobar'Returns true if the given string is a palindrome, false otherwise.
Convert the string to String.prototype.toLowerCase() and use String.prototype.replace() to remove non-alphanumeric characters from it.
Then, use the spread operator (...) to split the string into individual characters, Array.prototype.reverse(), String.prototype.join('') and compare it to the original, unreversed string, after converting it to String.prototype.toLowerCase().
const palindrome = str => {
const s = str.toLowerCase().replace(/[\W_]/g, '');
return s === [...s].reverse().join('');
};Examples
palindrome('taco cat'); // trueReturns the singular or plural form of the word based on the input number. If the first argument is an object, it will use a closure by returning a function that can auto-pluralize words that don't simply end in s if the supplied dictionary contains the word.
If num is either -1 or 1, return the singular form of the word. If num is any other number, return the plural form. Omit the third argument to use the default of the singular word + s, or supply a custom pluralized word when necessary. If the first argument is an object, utilize a closure by returning a function which can use the supplied dictionary to resolve the correct plural form of the word.
const pluralize = (val, word, plural = word + 's') => {
const _pluralize = (num, word, plural = word + 's') =>
[1, -1].includes(Number(num)) ? word : plural;
if (typeof val === 'object') return (num, word) => _pluralize(num, word, val[word]);
return _pluralize(val, word, plural);
};Examples
pluralize(0, 'apple'); // 'apples'
pluralize(1, 'apple'); // 'apple'
pluralize(2, 'apple'); // 'apples'
pluralize(2, 'person', 'people'); // 'people'
const PLURALS = {
person: 'people',
radius: 'radii'
};
const autoPluralize = pluralize(PLURALS);
autoPluralize(2, 'person'); // 'people'Removes non-printable ASCII characters.
Use a regular expression to remove non-printable ASCII characters.
const removeNonASCII = str => str.replace(/[^\x20-\x7E]/g, '');Examples
removeNonASCII('äÄçÇéÉêlorem-ipsumöÖÐþúÚ'); // 'lorem-ipsum'Reverses a string.
Use the spread operator (...) and Array.prototype.reverse() to reverse the order of the characters in the string.
Combine characters to get a string using String.prototype.join('').
const reverseString = str => [...str].reverse().join('');Examples
reverseString('foobar'); // 'raboof'Alphabetically sorts the characters in a string.
Use the spread operator (...), Array.prototype.sort() and String.localeCompare() to sort the characters in str, recombine using String.prototype.join('').
const sortCharactersInString = str => [...str].sort((a, b) => a.localeCompare(b)).join('');Examples
sortCharactersInString('cabbage'); // 'aabbceg'Splits a multiline string into an array of lines.
Use String.prototype.split() and a regular expression to match line breaks and create an array.
const splitLines = str => str.split(/\r?\n/);Examples
splitLines('This\nis a\nmultiline\nstring.\n'); // ['This', 'is a', 'multiline', 'string.' , '']Generates all permutations of a string (contains duplicates).
Use recursion.
For each letter in the given string, create all the partial permutations for the rest of its letters.
Use Array.prototype.map() to combine the letter with each partial permutation, then Array.prototype.reduce() to combine all permutations in one array.
Base cases are for string length equal to 2 or 1.
const stringPermutations = str => {
if (str.length <= 2) return str.length === 2 ? [str, str[1] + str[0]] : [str];
return str
.split('')
.reduce(
(acc, letter, i) =>
acc.concat(stringPermutations(str.slice(0, i) + str.slice(i + 1)).map(val => letter + val)),
[]
);
};Examples
stringPermutations('abc'); // ['abc','acb','bac','bca','cab','cba']Removes HTML/XML tags from string.
Use a regular expression to remove HTML/XML tags from a string.
const stripHTMLTags = str => str.replace(/<[^>]*>/g, '');Examples
stripHTMLTags('<p><em>lorem</em> <strong>ipsum</strong></p>'); // 'lorem ipsum'Converts a string to camelcase.
Break the string into words and combine them capitalizing the first letter of each word, using a regexp.
const toCamelCase = str => {
let s =
str &&
str
.match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
.map(x => x.slice(0, 1).toUpperCase() + x.slice(1).toLowerCase())
.join('');
return s.slice(0, 1).toLowerCase() + s.slice(1);
};Examples
toCamelCase('some_database_field_name'); // 'someDatabaseFieldName'
toCamelCase('Some label that needs to be camelized'); // 'someLabelThatNeedsToBeCamelized'
toCamelCase('some-javascript-property'); // 'someJavascriptProperty'
toCamelCase('some-mixed_string with spaces_underscores-and-hyphens'); // 'someMixedStringWithSpacesUnderscoresAndHyphens'Converts a string to kebab case.
Break the string into words and combine them adding - as a separator, using a regexp.
const toKebabCase = str =>
str &&
str
.match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
.map(x => x.toLowerCase())
.join('-');Examples
toKebabCase('camelCase'); // 'camel-case'
toKebabCase('some text'); // 'some-text'
toKebabCase('some-mixed_string With spaces_underscores-and-hyphens'); // 'some-mixed-string-with-spaces-underscores-and-hyphens'
toKebabCase('AllThe-small Things'); // "all-the-small-things"
toKebabCase('IAmListeningToFMWhileLoadingDifferentURLOnMyBrowserAndAlsoEditingSomeXMLAndHTML'); // "i-am-listening-to-fm-while-loading-different-url-on-my-browser-and-also-editing-xml-and-html"Converts a string to snake case.
Break the string into words and combine them adding _ as a separator, using a regexp.
const toSnakeCase = str =>
str &&
str
.match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
.map(x => x.toLowerCase())
.join('_');Examples
toSnakeCase('camelCase'); // 'camel_case'
toSnakeCase('some text'); // 'some_text'
toSnakeCase('some-mixed_string With spaces_underscores-and-hyphens'); // 'some_mixed_string_with_spaces_underscores_and_hyphens'
toSnakeCase('AllThe-small Things'); // "all_the_smal_things"
toSnakeCase('IAmListeningToFMWhileLoadingDifferentURLOnMyBrowserAndAlsoEditingSomeXMLAndHTML'); // "i_am_listening_to_fm_while_loading_different_url_on_my_browser_and_also_editing_some_xml_and_html"Converts a string to title case.
Break the string into words, using a regexp, and combine them capitalizing the first letter of each word and adding a whitespace between them.
const toTitleCase = str =>
str
.match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
.map(x => x.charAt(0).toUpperCase() + x.slice(1))
.join(' ');Examples
toTitleCase('some_database_field_name'); // 'Some Database Field Name'
toTitleCase('Some label that needs to be title-cased'); // 'Some Label That Needs To Be Title Cased'
toTitleCase('some-package-name'); // 'Some Package Name'
toTitleCase('some-mixed_string with spaces_underscores-and-hyphens'); // 'Some Mixed String With Spaces Underscores And Hyphens'Truncates a string up to a specified length.
Determine if the string's length is greater than num.
Return the string truncated to the desired length, with '...' appended to the end or the original string.
const truncateString = (str, num) =>
str.length > num ? str.slice(0, num > 3 ? num - 3 : num) + '...' : str;Examples
truncateString('boomerang', 7); // 'boom...'Unescapes escaped HTML characters.
Use String.prototype.replace() with a regex that matches the characters that need to be unescaped, using a callback function to replace each escaped character instance with its associated unescaped character using a dictionary (object).
const unescapeHTML = str =>
str.replace(
/&|<|>|'|"/g,
tag =>
({
'&': '&',
'<': '<',
'>': '>',
''': "'",
'"': '"'
}[tag] || tag)
);Examples
unescapeHTML('<a href="#">Me & you</a>'); // '<a href="#">Me & you</a>'Joins all given URL segments together, then normalizes the resulting URL.
Use String.prototype.join('/') to combine URL segments, then a series of String.prototype.replace() calls with various regexps to normalize the resulting URL (remove double slashes, add proper slashes for protocol, remove slashes before parameters, combine parameters with '&' and normalize first parameter delimiter).
const URLJoin = (...args) =>
args
.join('/')
.replace(/[\/]+/g, '/')
.replace(/^(.+):\//, '$1://')
.replace(/^file:/, 'file:/')
.replace(/\/(\?|&|#[^!])/g, '$1')
.replace(/\?/g, '&')
.replace('&', '?');Examples
URLJoin('http://www.google.com', 'a', '/b/cd', '?foo=123', '?bar=foo'); // 'http://www.google.com/a/b/cd?foo=123&bar=foo'Converts a given string into an array of words.
Use String.prototype.split() with a supplied pattern (defaults to non-alpha as a regexp) to convert to an array of strings. Use Array.prototype.filter() to remove any empty strings.
Omit the second argument to use the default regexp.
const words = (str, pattern = /[^a-zA-Z-]+/) => str.split(pattern).filter(Boolean);Examples
words('I love javaScript!!'); // ["I", "love", "javaScript"]
words('python, javaScript & coffee'); // ["python", "javaScript", "coffee"]Returns the native type of a value.
Returns lowercased constructor name of value, "undefined" or "null" if value is undefined or null.
const getType = v =>
v === undefined ? 'undefined' : v === null ? 'null' : v.constructor.name.toLowerCase();Examples
getType(new Set([1, 2, 3])); // 'set'Checks if the provided value is of the specified type.
Ensure the value is not undefined or null using Array.prototype.includes(), and compare the constructor property on the value with type to check if the provided value is of the specified type.
const is = (type, val) => ![, null].includes(val) && val.constructor === type;Examples
is(Array, [1]); // true
is(ArrayBuffer, new ArrayBuffer()); // true
is(Map, new Map()); // true
is(RegExp, /./g); // true
is(Set, new Set()); // true
is(WeakMap, new WeakMap()); // true
is(WeakSet, new WeakSet()); // true
is(String, ''); // true
is(String, new String('')); // true
is(Number, 1); // true
is(Number, new Number(1)); // true
is(Boolean, true); // true
is(Boolean, new Boolean(true)); // trueChecks if the provided argument is array-like (i.e. is iterable).
Check if the provided argument is not null and that its Symbol.iterator property is a function.
const isArrayLike = obj => obj != null && typeof obj[Symbol.iterator] === 'function';Examples
isArrayLike(document.querySelectorAll('.className')); // true
isArrayLike('abc'); // true
isArrayLike(null); // falseChecks if the given argument is a native boolean element.
Use typeof to check if a value is classified as a boolean primitive.
const isBoolean = val => typeof val === 'boolean';Examples
isBoolean(null); // false
isBoolean(false); // trueReturns true if the a value is an empty object, collection, has no enumerable properties or is any type that is not considered a collection.
Check if the provided value is null or if its length is equal to 0.
const isEmpty = val => val == null || !(Object.keys(val) || val).length;Examples
isEmpty([]); // true
isEmpty({}); // true
isEmpty(''); // true
isEmpty([1, 2]); // false
isEmpty({ a: 1, b: 2 }); // false
isEmpty('text'); // false
isEmpty(123); // true - type is not considered a collection
isEmpty(true); // true - type is not considered a collectionChecks if the given argument is a function.
Use typeof to check if a value is classified as a function primitive.
const isFunction = val => typeof val === 'function';Examples
isFunction('x'); // false
isFunction(x => x); // trueReturns true if the specified value is null or undefined, false otherwise.
Use the strict equality operator to check if the value and of val are equal to null or undefined.
const isNil = val => val === undefined || val === null;Examples
isNil(null); // true
isNil(undefined); // trueReturns true if the specified value is null, false otherwise.
Use the strict equality operator to check if the value and of val are equal to null.
const isNull = val => val === null;Examples
isNull(null); // trueChecks if the given argument is a number.
Use typeof to check if a value is classified as a number primitive.
To safeguard against NaN, check if val === val (as NaN has a typeof equal to number and is the only value not equal to itself).
const isNumber = val => typeof val === 'number' && val === val;Examples
isNumber(1); // true
isNumber('1'); // false
isNumber(NaN); // falseReturns a boolean determining if the passed value is an object or not.
Uses the Object constructor to create an object wrapper for the given value.
If the value is null or undefined, create and return an empty object. Οtherwise, return an object of a type that corresponds to the given value.
const isObject = obj => obj === Object(obj);Examples
isObject([1, 2, 3, 4]); // true
isObject([]); // true
isObject(['Hello!']); // true
isObject({ a: 1 }); // true
isObject({}); // true
isObject(true); // falseChecks if a value is object-like.
Check if the provided value is not null and its typeof is equal to 'object'.
const isObjectLike = val => val !== null && typeof val === 'object';Examples
isObjectLike({}); // true
isObjectLike([1, 2, 3]); // true
isObjectLike(x => x); // false
isObjectLike(null); // falseChecks if the provided value is an object created by the Object constructor.
Check if the provided value is truthy, use typeof to check if it is an object and Object.constructor to make sure the constructor is equal to Object.
const isPlainObject = val => !!val && typeof val === 'object' && val.constructor === Object;Examples
isPlainObject({ a: 1 }); // true
isPlainObject(new Map()); // falseReturns a boolean determining if the passed value is primitive or not.
Create an object from val and compare it with val to determine if the passed value is primitive (i.e. not equal to the created object).
const isPrimitive = val => Object(val) !== val;Examples
isPrimitive(null); // true
isPrimitive(50); // true
isPrimitive('Hello!'); // true
isPrimitive(false); // true
isPrimitive(Symbol()); // true
isPrimitive([]); // falseReturns true if an object looks like a Promise, false otherwise.
Check if the object is not null, its typeof matches either object or function and if it has a .then property, which is also a function.
const isPromiseLike = obj =>
obj !== null &&
(typeof obj === 'object' || typeof obj === 'function') &&
typeof obj.then === 'function';Examples
isPromiseLike({
then: function() {
return '';
}
}); // true
isPromiseLike(null); // false
isPromiseLike({}); // falseChecks if the given argument is a string. Only works for string primitives.
Use typeof to check if a value is classified as a string primitive.
const isString = val => typeof val === 'string';Examples
isString('10'); // trueChecks if the given argument is a symbol.
Use typeof to check if a value is classified as a symbol primitive.
const isSymbol = val => typeof val === 'symbol';Examples
isSymbol(Symbol('x')); // trueReturns true if the specified value is undefined, false otherwise.
Use the strict equality operator to check if the value and of val are equal to undefined.
const isUndefined = val => val === undefined;Examples
isUndefined(undefined); // trueChecks if the provided string is a valid JSON.
Use JSON.parse() and a try... catch block to check if the provided string is a valid JSON.
const isValidJSON = str => {
try {
JSON.parse(str);
return true;
} catch (e) {
return false;
}
};Examples
isValidJSON('{"name":"Adam","age":20}'); // true
isValidJSON('{"name":"Adam",age:"20"}'); // false
isValidJSON(null); // trueCasts the provided value as an array if it's not one.
Use Array.prototype.isArray() to determine if val is an array and return it as-is or encapsulated in an array accordingly.
const castArray = val => (Array.isArray(val) ? val : [val]);Examples
castArray('foo'); // ['foo']
castArray([1]); // [1]Clones a regular expression.
Use new RegExp(), RegExp.source and RegExp.flags to clone the given regular expression.
const cloneRegExp = regExp => new RegExp(regExp.source, regExp.flags);Examples
const regExp = /lorem ipsum/gi;
const regExp2 = cloneRegExp(regExp); // /lorem ipsum/giReturns the first non-null/undefined argument.
Use Array.prototype.find() to return the first non null/undefined argument.
const coalesce = (...args) => args.find(_ => ![undefined, null].includes(_));Examples
coalesce(null, undefined, '', NaN, 'Waldo'); // ""Returns a customized coalesce function that returns the first argument that returns true from the provided argument validation function.
Use Array.prototype.find() to return the first argument that returns true from the provided argument validation function.
const coalesceFactory = valid => (...args) => args.find(valid);Examples
const customCoalesce = coalesceFactory(_ => ![null, undefined, '', NaN].includes(_));
customCoalesce(undefined, null, NaN, '', 'Waldo'); // "Waldo"Extends a 3-digit color code to a 6-digit color code.
Use Array.prototype.map(), String.prototype.split() and Array.prototype.join() to join the mapped array for converting a 3-digit RGB notated hexadecimal color-code to the 6-digit form.
Array.prototype.slice() is used to remove # from string start since it's added once.
const extendHex = shortHex =>
'#' +
shortHex
.slice(shortHex.startsWith('#') ? 1 : 0)
.split('')
.map(x => x + x)
.join('');Examples
extendHex('#03f'); // '#0033ff'
extendHex('05a'); // '#0055aa'Returns an object containing the parameters of the current URL.
Use String.match() with an appropriate regular expression to get all key-value pairs, Array.prototype.reduce() to map and combine them into a single object.
Pass location.search as the argument to apply to the current url.
const getURLParameters = url =>
(url.match(/([^?=&]+)(=([^&]*))/g) || []).reduce(
(a, v) => ((a[v.slice(0, v.indexOf('='))] = v.slice(v.indexOf('=') + 1)), a),
{}
);Examples
getURLParameters('http://url.com/page?name=Adam&surname=Smith'); // {name: 'Adam', surname: 'Smith'}
getURLParameters('google.com'); // {}Converts a color code to a rgb() or rgba() string if alpha value is provided.
Use bitwise right-shift operator and mask bits with & (and) operator to convert a hexadecimal color code (with or without prefixed with #) to a string with the RGB values. If it's 3-digit color code, first convert to 6-digit version. If an alpha value is provided alongside 6-digit hex, give rgba() string in return.
const hexToRGB = hex => {
let alpha = false,
h = hex.slice(hex.startsWith('#') ? 1 : 0);
if (h.length === 3) h = [...h].map(x => x + x).join('');
else if (h.length === 8) alpha = true;
h = parseInt(h, 16);
return (
'rgb' +
(alpha ? 'a' : '') +
'(' +
(h >>> (alpha ? 24 : 16)) +
', ' +
((h & (alpha ? 0x00ff0000 : 0x00ff00)) >>> (alpha ? 16 : 8)) +
', ' +
((h & (alpha ? 0x0000ff00 : 0x0000ff)) >>> (alpha ? 8 : 0)) +
(alpha ? `, ${h & 0x000000ff}` : '') +
')'
);
};Examples
hexToRGB('#27ae60ff'); // 'rgba(39, 174, 96, 255)'
hexToRGB('27ae60'); // 'rgb(39, 174, 96)'
hexToRGB('#fff'); // 'rgb(255, 255, 255)'Makes a GET request to the passed URL.
Use XMLHttpRequest web api to make a get request to the given url.
Handle the onload event, by calling the given callback the responseText.
Handle the onerror event, by running the provided err function.
Omit the third argument, err, to log errors to the console's error stream by default.
const httpGet = (url, callback, err = console.error) => {
const request = new XMLHttpRequest();
request.open('GET', url, true);
request.onload = () => callback(request.responseText);
request.onerror = () => err(request);
request.send();
};Examples
httpGet(
'https://jsonplaceholder.typicode.com/posts/1',
console.log
); /*
Logs: {
"userId": 1,
"id": 1,
"title": "sunt aut facere repellat provident occaecati excepturi optio reprehenderit",
"body": "quia et suscipit\nsuscipit recusandae consequuntur expedita et cum\nreprehenderit molestiae ut ut quas totam\nnostrum rerum est autem sunt rem eveniet architecto"
}
*/Makes a POST request to the passed URL.
Use XMLHttpRequest web api to make a post request to the given url.
Set the value of an HTTP request header with setRequestHeader method.
Handle the onload event, by calling the given callback the responseText.
Handle the onerror event, by running the provided err function.
Omit the third argument, data, to send no data to the provided url.
Omit the fourth argument, err, to log errors to the console's error stream by default.
const httpPost = (url, data, callback, err = console.error) => {
const request = new XMLHttpRequest();
request.open('POST', url, true);
request.setRequestHeader('Content-type', 'application/json; charset=utf-8');
request.onload = () => callback(request.responseText);
request.onerror = () => err(request);
request.send(data);
};Examples
const newPost = {
userId: 1,
id: 1337,
title: 'Foo',
body: 'bar bar bar'
};
const data = JSON.stringify(newPost);
httpPost(
'https://jsonplaceholder.typicode.com/posts',
data,
console.log
); /*
Logs: {
"userId": 1,
"id": 1337,
"title": "Foo",
"body": "bar bar bar"
}
*/
httpPost(
'https://jsonplaceholder.typicode.com/posts',
null, // does not send a body
console.log
); /*
Logs: {
"id": 101
}
*/Determines if the current runtime environment is a browser so that front-end modules can run on the server (Node) without throwing errors.
Use Array.prototype.includes() on the typeof values of both window and document (globals usually only available in a browser environment unless they were explicitly defined), which will return true if one of them is undefined.
typeof allows globals to be checked for existence without throwing a ReferenceError.
If both of them are not undefined, then the current environment is assumed to be a browser.
const isBrowser = () => ![typeof window, typeof document].includes('undefined');Examples
isBrowser(); // true (browser)
isBrowser(); // false (Node)Returns the index of the function in an array of functions which executed the fastest.
Use Array.prototype.map() to generate an array where each value is the total time taken to execute the function after iterations times. Use the difference in performance.now() values before and after to get the total time in milliseconds to a high degree of accuracy.
Use Math.min() to find the minimum execution time, and return the index of that shortest time which corresponds to the index of the most performant function.
Omit the second argument, iterations, to use a default of 10,000 iterations. The more iterations, the more reliable the result but the longer it will take.
const mostPerformant = (fns, iterations = 10000) => {
const times = fns.map(fn => {
const before = performance.now();
for (let i = 0; i < iterations; i++) fn();
return performance.now() - before;
});
return times.indexOf(Math.min(...times));
};Examples
mostPerformant([
() => {
// Loops through the entire array before returning `false`
[1, 2, 3, 4, 5, 6, 7, 8, 9, '10'].every(el => typeof el === 'number');
},
() => {
// Only needs to reach index `1` before returning false
[1, '2', 3, 4, 5, 6, 7, 8, 9, 10].every(el => typeof el === 'number');
}
]); // 1Creates a function that gets the argument at index n. If n is negative, the nth argument from the end is returned.
Use Array.prototype.slice() to get the desired argument at index n.
const nthArg = n => (...args) => args.slice(n)[0];Examples
const third = nthArg(2);
third(1, 2, 3); // 3
third(1, 2); // undefined
const last = nthArg(-1);
last(1, 2, 3, 4, 5); // 5Returns a query string generated from the key-value pairs of the given object.
Use Array.prototype.reduce() on Object.entries(queryParameters) to create the query string.
Determine the symbol to be either ? or & based on the index and concatenate val to queryString only if it's a string.
Return the queryString or an empty string when the queryParameters are falsy.
const objectToQueryString = queryParameters => {
return queryParameters
? Object.entries(queryParameters).reduce((queryString, [key, val], index) => {
const symbol = index === 0 ? '?' : '&';
queryString += typeof val === 'string' ? `${symbol}${key}=${val}` : '';
return queryString;
}, '')
: '';
};Examples
objectToQueryString({ page: '1', size: '2kg', key: undefined }); // '?page=1&size=2kg'Parse an HTTP Cookie header string and return an object of all cookie name-value pairs.
Use String.prototype.split(';') to separate key-value pairs from each other.
Use Array.prototype.map() and String.prototype.split('=') to separate keys from values in each pair.
Use Array.prototype.reduce() and decodeURIComponent() to create an object with all key-value pairs.
const parseCookie = str =>
str
.split(';')
.map(v => v.split('='))
.reduce((acc, v) => {
acc[decodeURIComponent(v[0].trim())] = decodeURIComponent(v[1].trim());
return acc;
}, {});Examples
parseCookie('foo=bar; equation=E%3Dmc%5E2'); // { foo: 'bar', equation: 'E=mc^2' }Converts a number in bytes to a human-readable string.
Use an array dictionary of units to be accessed based on the exponent.
Use Number.toPrecision() to truncate the number to a certain number of digits.
Return the prettified string by building it up, taking into account the supplied options and whether it is negative or not.
Omit the second argument, precision, to use a default precision of 3 digits.
Omit the third argument, addSpace, to add space between the number and unit by default.
const prettyBytes = (num, precision = 3, addSpace = true) => {
const UNITS = ['B', 'KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];
if (Math.abs(num) < 1) return num + (addSpace ? ' ' : '') + UNITS[0];
const exponent = Math.min(Math.floor(Math.log10(num < 0 ? -num : num) / 3), UNITS.length - 1);
const n = Number(((num < 0 ? -num : num) / 1000 ** exponent).toPrecision(precision));
return (num < 0 ? '-' : '') + n + (addSpace ? ' ' : '') + UNITS[exponent];
};Examples
prettyBytes(1000); // "1 KB"
prettyBytes(-27145424323.5821, 5); // "-27.145 GB"
prettyBytes(123456789, 3, false); // "123MB"Generates a random hexadecimal color code.
Use Math.random to generate a random 24-bit(6x4bits) hexadecimal number. Use bit shifting and then convert it to an hexadecimal String using toString(16).
const randomHexColorCode = () => {
let n = (Math.random() * 0xfffff * 1000000).toString(16);
return '#' + n.slice(0, 6);
};Examples
randomHexColorCode(); // "#e34155"Converts the values of RGB components to a color code.
Convert given RGB parameters to hexadecimal string using bitwise left-shift operator (<<) and toString(16), then String.padStart(6,'0') to get a 6-digit hexadecimal value.
const RGBToHex = (r, g, b) => ((r << 16) + (g << 8) + b).toString(16).padStart(6, '0');Examples
RGBToHex(255, 165, 1); // 'ffa501'Serialize a cookie name-value pair into a Set-Cookie header string.
Use template literals and encodeURIComponent() to create the appropriate string.
const serializeCookie = (name, val) => `${encodeURIComponent(name)}=${encodeURIComponent(val)}`;Examples
serializeCookie('foo', 'bar'); // 'foo=bar'Measures the time taken by a function to execute.
Use console.time() and console.timeEnd() to measure the difference between the start and end times to determine how long the callback took to execute.
const timeTaken = callback => {
console.time('timeTaken');
const r = callback();
console.timeEnd('timeTaken');
return r;
};Examples
timeTaken(() => Math.pow(2, 10)); // 1024, (logged): timeTaken: 0.02099609375msTake a number and return specified currency formatting.
Use Intl.NumberFormat to enable country / currency sensitive formatting.
const toCurrency = (n, curr, LanguageFormat = undefined) =>
Intl.NumberFormat(LanguageFormat, { style: 'currency', currency: curr }).format(n);Examples
toCurrency(123456.789, 'EUR'); // €123,456.79 | currency: Euro | currencyLangFormat: Local
toCurrency(123456.789, 'USD', 'en-us'); // $123,456.79 | currency: US Dollar | currencyLangFormat: English (United States)
toCurrency(123456.789, 'USD', 'fa'); // ۱۲۳٬۴۵۶٫۷۹ $ | currency: US Dollar | currencyLangFormat: Farsi
toCurrency(322342436423.2435, 'JPY'); // ¥322,342,436,423 | currency: Japanese Yen | currencyLangFormat: Local
toCurrency(322342436423.2435, 'JPY', 'fi'); // 322 342 436 423 ¥ | currency: Japanese Yen | currencyLangFormat: FinnishUse toLocaleString() to convert a float-point arithmetic to the Decimal mark form. It makes a comma separated string from a number.
const toDecimalMark = num => num.toLocaleString('en-US');Examples
toDecimalMark(12305030388.9087); // "12,305,030,388.909"Adds an ordinal suffix to a number.
Use the modulo operator (%) to find values of single and tens digits.
Find which ordinal pattern digits match.
If digit is found in teens pattern, use teens ordinal.
const toOrdinalSuffix = num => {
const int = parseInt(num),
digits = [int % 10, int % 100],
ordinals = ['st', 'nd', 'rd', 'th'],
oPattern = [1, 2, 3, 4],
tPattern = [11, 12, 13, 14, 15, 16, 17, 18, 19];
return oPattern.includes(digits[0]) && !tPattern.includes(digits[1])
? int + ordinals[digits[0] - 1]
: int + ordinals[3];
};Examples
toOrdinalSuffix('123'); // "123rd"Returns true if the given value is a number, false otherwise.
Use !isNaN() in combination with parseFloat() to check if the argument is a number.
Use isFinite() to check if the number is finite.
Use Number() to check if the coercion holds.
const validateNumber = n => !isNaN(parseFloat(n)) && isFinite(n) && Number(n) == n;Examples
validateNumber('10'); // trueReturns true if the string is y/yes or false if the string is n/no.
Use RegExp.test() to check if the string evaluates to y/yes or n/no.
Omit the second argument, def to set the default answer as no.
const yesNo = (val, def = false) =>
/^(y|yes)$/i.test(val) ? true : /^(n|no)$/i.test(val) ? false : def;Examples
yesNo('Y'); // true
yesNo('yes'); // true
yesNo('No'); // false
yesNo('Foo', true); // true Angelos Chalaris |
 Felix Wu |
 Stefan Feješ |
 King David Martins |
 Soorena Soleimani |
|---|---|---|---|---|
 Elder Henrique Souza |
 Robert Mennell |
 atomiks |
Logos made by Angelos Chalaris are licensed under the MIT license. This README is built using markdown-builder.