Javascript Cheatsheet
Basics
Boolean type
Declaration
const someBoolean = false
const someBoolean = trueNumber type
Declaration
const number = 19.91Functions
// Check if variable is not a number
const someBoolean = isNaN('someString') // true
const someBoolean = isNaN(19.91) // false
const someBoolean = isNaN('19.91') // false
// Convert number/string to float number (with decimals)
const someNumber = parseFloat('15.92') // 15.92
// Convert number/string to integer number (without decimals)
const someNumber = parseInt(number) // 19Methods
// Convert number to string
const someString = number.toString() // '19.91'
// Convert number to string
const someString = number.toString() // '19.91'
// Define amount of decimals
const someString = number.toFixed(1) // '19.9'
const someString = number.toFixed(3) // '19.910'String type
Declaration
const string = 'The king is my father'
const string = "The king is my father"
// String templates
const who = 'father'
const string = `The king is my ${who}` // 'The king is my father'Methods
// Check if string starts with substring (case sensitive)
const someBoolean = string.startsWith('The') // true
const someBoolean = string.startsWith('the') // false
// Check if string ends with substring (case sensitive)
const someBoolean = string.endsWith('father') // true
// Check if string includes substring
const someBoolean = string.includes('king is') // true
// Replace a substring by other substring
const someString = string.replaceAll('my', 'your') // 'The king is your father'
// Make string lower case
const someString = string.toLowerCase() // 'the king is my father'
// Make string upper case
const someString = string.toUpperCase() // 'THE KING IS MY FATHER'
// Remove start/end spaces
const stringWithSpaces = ' some string with spaces '
const someString = stringWithSpaces.trim() // 'string with spaces'
// Get length of string
const someNumber = string.length // 21
// Split/get list from string given a substring as separator
const someArray = string.split(' ') // ['The', 'king', 'is', 'my', 'father']Object type
Declaration
const object = {
name: 'John',
age: 25,
}Methods
// Get property value
// Fixed key
const someNumber = object.age // 25
// Variable key
const key = 'age'
const someNumber = object[key] // 25
// Add property
// Fixed key
object.dni = '123-45-6789'
// Dynamic key
const key = 'dni'
object[key] = '123-45-6789'
// {
// name: 'John',
// age: 25,
// dni: '123-45-6789',
// }
// Change property value
// Fixed key
object.name = 'John Doe'
// Variable key
const key = 'name'
object[key] = 'John Doe'
// {
// name: 'John Doe',
// age: 25,
// }
// Delete property
// Fixed key
delete object.age
// Variable key
const key = 'age'
delete object[key]
// {
// name: 'John Doe',
// }
// Check if property exists in object
const key = 'name'
key in object // trueFunctions
// Get keys as list
const someArray = Object.keys(object) // ['name', 'age']
// Get values as list
const someArray = Object.values(object) // ['John', 25]
// Get keys and values as list
const someArray = Object.entries(object) // [['name', 'John'], ['age', 25]]Array type
Declaration
const array = ['País Vasco', 'Islas Cíes', 'Cantabria', 'Islas Canarias']
// Create array of some value given a length
const value = null
const arrayOfValueGivenALength = Array(3).fill(value) // [null, null, null]Methods
// Get length of array
const someNumber = array.length // 4
// Add element to array (Modifies itself)
array.push('La Alpujarra')
// ['País Vasco', 'Islas Cíes', 'Cantabria', 'Islas Canarias', 'La Alpujarra']
// Get element of aray given a position
// Fixed key
const someString = array[1] // 'Islas Cíes'
// Variable key
const position = 1
const someString = array[position] // 'Islas Cíes'
// Ireate array / Execute function for each element
array.forEach((value, position) => {
console.log(`${value} is at position ${position}`)
})
// 'País Vasco is at position 0',
// 'Islas Cíes is at position 1',
// 'Cantabria is at position 2',
// 'Islas Canarias is at position 3'
// Map to new values given a function
const someArray = array.map((value, position) => {
return `${value} is at position ${position}`
})
// [
// 'País Vasco is at position 0',
// 'Islas Cíes is at position 1',
// 'Cantabria is at position 2',
// 'Islas Canarias is at position 3'
// ]
// Filter array given a function. Only 'true' evaluations are added to the result
const someArray = array.filter((value, position) => {
return value.startsWith('Islas') // boolean
})
// [
// 'Islas Cíes',
// 'Islas Canarias'
// ]
// Find first element of an array that satisfies a condition.
const someString = array.find((value, position) => {
return value.startsWith('Islas')
})
// 'Islas Cíes'
// Check if SOME value of the array satisfies a condition.
const someBoolean = array.some((value, position) => {
return value.startsWith('Islas')
})
// true
// Check if EVERY value of the array value satisfies a condition.
const someBoolean = array.every((value, position) => {
return value.startsWith('Islas')
})
// false
// Check if array includes a value
const someBoolean = array.includes('Cantabria') // true
// Check position of a value in the array (only first one)
const someNumber = array.indexOf('Cantabria') // 2
// Join array of strings in single string (giving a separator)
const someString = array.join('-') // 'País Vasco-Islas Cíes-Cantabria-Islas Canarias'
// Reverse order of array (Modifies itself)
array.reverse() // ['Islas Canarias', 'Cantabria', 'Islas Cíes', 'País Vasco']Order functions
// Order from min to max (Modifies itself)
const array = [40, 100, 1, 5, 25, 10]
array.sort((a, b) => a - b) // [1, 5, 10, 25, 40, 100]
// Order from max to min (Modifies itself)
const array = [40, 100, 1, 5, 25, 10]
array.sort((a, b) => b - a) // [100, 40, 25, 10, 5, 1]
// Order alphabetically (Modifies itself)
const array = ['País Vasco', 'Islas Cíes', 'Cantabria', 'Islas Canarias']
array.sort((a, b) => a.localeCompare(b)) // ['Cantabria', 'Islas Canarias', 'Islas Cíes', 'País Vasco']