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AJAX Advantages and Disadvantages JavaScript

JavaScript Let vs Var - When should let be used over var?

Let Keyword -
The “let” keyword declares a block-scoped variable. Let and var declared outside any block, both are global.
The “let” keyword allows you to declare a variable that are limited in scope to the block, statement, or expression on which it’s used.

You can assign values to the variables when you declare them or later in your script. If you do not initialize your variable in the “let” statement, it is automatically assigned the JavaScript value.

Let variables cannot be re-declared while var variable can be re-declared in the same scope.
A variable declared by “let” can’t be used before its declaration or an error will result.

A variable declaration with “let” is helpful when we want the scope of a variable to be limited to statement or expression level.

Let gives you the privilege to declare variables that are limited in scope to the block, statement of expression unlike var.

The “let” statement declares a local variable in a block scope. It is similar to var, in that we can optionally initialize the variable.

Example looks like –
let a = 10;

The “let” statement allows you to create a variable with the scope limited to the block on which it is used.

It is similar to the variable we declare in other languages like Java, .NET, etc.

Var Keyword -
The JavaScript variables statement is used to declare a variable and, optionally, we can initialize the value of that variable.

Var is keyword which defines a variable globally regardless of block scope.

Example looks like –
var a = 10;
The var keyword can be used everywhere. You can put in all kinds of information like strings, Int, Object, Array and Function.

Variable declarations are processed before the execution of the code.
The scope of a JavaScript variable declared with var is its current execution context.
The scope of a JavaScript variable declared outside the function is global.

For example 1,
function variable_Let_Var() {
  console.log(i); // i is undefined due to hoisting
  //console.log(j); // ReferenceError: j is not defined

  for( var i = 0; i < 3; i++ ) {
    console.log(i); //Output- 0, 1, 2

  console.log(i); // Output - 3
  // console.log(j); //ReferenceError: j is not defined

  for( let j = 0; j < 3; j++ ) {

  console.log(i); // 3
  // console.log(j); // ReferenceError: j is not defined

For example 2 -
var a = 10;
var b = 20;

if (a === 10) {
  var a = 11; // The scope is global
  let b = 22; // The scope is inside the if-block

  console.log(a);  // 11
  console.log(b);  // 22

console.log(a); // 11
console.log(b); // 20

For example 3 -
function test_variable(){
  var a = 33;
  if (true) {
     let a = (a + 55); // ReferenceError : [ts] Block-scoped variable 'a' used before its declaration.

Should I replace var with let?
No, let is the new block scoping var.
The “var” variable scope is function level but “let” variable is scoped in its enclosing block - ({}).

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