for defining variables in JavaScript we have let and const for variables. the let could change him values but const variables could not change and you could not assign value after definition
you could define Strings in 3 way "quotation", ‘quotation’,quotation
for string interpolation you should use `` with ${} pattern ,=> Abolfazl ${variables}
the syntax of conditions in JavaScript is like this : if(condition){} , the else is like this if(condition){}else{} . the else if syntax is like this if(condition){}else if(condition){} .
for commenting in JS we use // for commenting
When we define a variable in JavaScript and do not assign a value, it is considered undefined. On the other hand, null is used to indicate that a variable has no value.
JavaScript has three logical operators: &&, ||, and !.
- The
&&operator returnstrueif both operands aretrue, andfalseotherwise. - The
||operator returnstrueif at least one of the operands istrue, andfalseotherwise. - The
!operator returns the opposite of the operand's boolean value (truebecomesfalse, andfalsebecomestrue).
These operators are often used in conjunction with conditional statements to create more complex logic.
JavaScript has several math operators that can be used in conditions, including:
<: less than>: greater than<=: less than or equal to>=: greater than or equal to===: equal to (compares value and type)!==: not equal to (compares value and type)
These operators can be used to compare variables or values in conditional statements. For example:
let x = 5;
if (x < 10) {
console.log("x is less than 10");
}
In this example, the condition x < 10 evaluates to true, so the code inside the if statement is executed and "x is less than 10" is logged to the console.
Note that the === and !== operators compare both the value and the type of the operands. This means that "5" === 5 would evaluate to false, because the left operand is a string and the right operand is a number.
for creating functions in JavaScript we create it with function key word then the name of function , the functions could have inputs. the parameters could be the input of functions
function sayHello(parameter){
console.log(parameter);
}for checking that value is exists or not we could use ! operator in out code lets see example
if(!user){
// condition would excute if there is no user providede to us
}for doing the if checking of available value we could also use this kind of checking
user=user||'empty state is valid';here if user did not provided the value would be other string
For checking if variables have null or undefined in JavaScript as we do in dart we could use ?? operator in our code , note that this just check if value is null or undefined and it dose not do any more
user=user??'the state is only valid when the user is null or undefinded'the expersion functions are kind of functions which they could place in variables like this
let myVar= function (){
alert('my functuion')
}
sayHi();if the () are not placed in-front of the function is like that it dos not run so we could run it by placing () in-front of the variable , the returned value from function would stored on the variables , and it could not before initializing usage
the arrow functions is like that experssion functions in js but the way of implementing is different . like this.
let myFunction=(param,param2)=>{
console.log()
}for defining Arrays in Js we could use let or const like this
let users=[];
const podcasts=[];let users=[];
users.push('abolfazl')let users=['abolfazl','mostafa']
user.pop()
// result => ['abolfazl']let users=['abolfazl','mostafa']
users.shift()
// users =>['mostafa']let users=['abolfazl','mostafa']
users.unshift('asma')
//result=>['asma','abolfazl','mostafa']for creating loops inside the the Arrays in JavaScript in normal way we could do this way.
let users=['abolfazl','mostafa'];
for(let user of users){
console.log(user);
}
for(let i=0;i<users.length;i++){
console.log(users[i]);
}but there is other for in JavaScript called for of that is common in any programming language.
for...of and for...in are two types of loops used in JavaScript to iterate over collections such as arrays and objects.
The for...of loop allows you to iterate over the values in an array or other iterable object. It has the following syntax:
for (let value of iterable) {
// code to execute for each value
}Here, iterable is the collection of values you want to iterate over, and value is a variable that will be assigned to each value in turn. The loop will execute once for each value in the iterable.
For example, the following code uses a for...of loop to log each value in an array to the console:
let numbers = [1, 2, 3];
for (let number of numbers) {
console.log(number);
}
// Output:
// 1
// 2
// 3The for...in loop allows you to iterate over the properties of an object. It has the following syntax:
for (let property in object) {
// code to execute for each property
}
Here, object is the object whose properties you want to iterate over, and property is a variable that will be assigned to each property in turn. The loop will execute once for each property in the object.
For example, the following code uses a for...in loop to log each property of an object to the console:
let person = { name: "Alice", age: 30 };
for (let property in person) {
console.log(property + ": " + person[property]);
}
// Output:
// name: Alice
// age: 30
Note that the order in which properties are iterated over is not guaranteed to be the same as the order in which they were defined in the object. If you need to iterate over properties in a specific order, you should use an array instead.
The splice() method in JavaScript is used to add or remove items from an array at a specific index. It takes in two required arguments: the starting index and the number of items to remove. It can also take in additional arguments to specify items to add to the array at the specified index. If no items are specified to add, splice() will simply remove the items specified by the starting index and number of items to remove.
Example of using the splice() method:
let numbers = [1, 2, 3, 4, 5];
numbers.splice(2, 1); // remove the third item (index 2)
// numbers is now [1, 2, 4, 5]
numbers.splice(1, 0, 6, 7); // add two items (6, 7) at index 1
// numbers is now [1, 6, 7, 2, 4, 5]
for adding value with splice method first we say which index we want to insert inside (1)
then we say how many to remove (1,0) then we specify the items which we want to add (1,0,6,7) . For removing that we just specify the index where to start then we specify how many we want to remove
The forEach() method in JavaScript is used to iterate over an array and execute a function for each element. It takes in a single argument, which is a function to execute for each element of the array. The function takes in three arguments: the current element, the index of the current element, and the array being iterated over.
Here is an example of using the forEach() method to log each element of an array to the console:
let numbers = [1, 2, 3];
numbers.forEach(function(number) {
console.log(number);
});
// Output:
// 1
// 2
// 3
In this example, the forEach() method is called on the numbers array and passed a function that logs each element to the console. The function takes a single parameter, number, which represents the current element of the array being iterated over.
Note that the forEach() method does not modify the original array. If you need to modify the array, you should use a different method, such as map() or filter().
For finding specific item in list in js we could use find method like this , if not find would return undefinded , if there is multiply items in list it would return the first item
let users=['abolfazl','mostafa','reza']
let user=users.find((item)=>{
if(item =='abolfazl'){
return item
}
})
let user=users.find(item=>item=='abolfazl');The filter method used for getting items with the some shared things in Array. this method is like the where method of dart Arrays which you could create a list of items with specified item.
Here is an example of using the filter() method to create a new array of numbers greater than 3:
let numbers = [1, 2, 3, 4, 5];
let filteredNumbers = numbers.filter(function(number) {
return number > 3;
});
// filteredNumbers is now [4, 5]
In this example, the filter() method is called on the numbers array and passed a function that returns true if the current element is greater than 3. The method returns a new array containing only the elements that meet the condition.
Note that the filter() method does not modify the original array. If you need to modify the array, you should use a different method, such as map() or splice().
To search for and determine if a specific item is available in a list, you can use the includes method. it like the contains method of Dart.
const skills=['nodeJs','Mongo','Dart']
const hasNodeJs=skills.includes('nodeJs')
// result => trueThis method returns true if there are any available matched items, else it returns false. this method is flexible for making condition for items
const skills=['nodeJs','dart','flutter']
const available=skills.some(item=>item=='nodeJs')This method returns true if every item of Array match the condition we determine for it
const skill=['js','dart','htm','web'];
const everyItem=skill.every(item=>item=='js')// fales
const isNotEmptyArray=skill.every(item=>item.lenght>2)//trueThis method returns the index of element in array , this method is similar to indexOf method in dart. and we could make condition for it
const skills=['js','dart','pythond','flutter']
const indexOfJs=skills.findIndex(item=>item=='js')//0This method is both important and useful. If you want to create a new Array from an existing Array without changing the default values, you can use map(). This is similar to Dart's map function.
const numbers=[1,2,3,4,5]
const doubledNumber=numbers.map((item)=>item*2)// [2,4,6,8,10]