let vs var vs const in Javascript

 

Block Scope

Before ES6 (2015), JavaScript did not have Block Scope.

JavaScript had Global Scope and Function Scope.

ES6 introduced the two new JavaScript keywords: let and const.

These two keywords provided Block Scope in JavaScript:

Example

Variables declared inside a { } block cannot be accessed from outside the block:

{
  let x = 2;
}
// x can NOT be used here

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Global Scope

Variables declared with the var always have Global Scope.

Variables declared with the var keyword can NOT have block scope:

Example

Variables declared with varinside a { } block can be accessed from outside the block:

{
  var x = 2;
}
// x CAN be used here

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Cannot be Redeclared

Variables defined with let can not be redeclared.

You can not accidentally redeclare a variable declared with let.

With let you can not do this:

let x = "John Doe";

let x = 0;

Variables defined with var can be redeclared.

With var you can do this:

var x = "John Doe";

var x = 0;

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Redeclaring Variables

Redeclaring a variable using the var keyword can impose problems.

Redeclaring a variable inside a block will also redeclare the variable outside the block:

Example

var x = 10;
// Here x is 10

{
var x = 2;
// Here x is 2
}

// Here x is 2

Try it Yourself »

Redeclaring a variable using the let keyword can solve this problem.

Redeclaring a variable inside a block will not redeclare the variable outside the block:

Example

let x = 10;
// Here x is 10

{
let x = 2;
// Here x is 2
}

// Here x is 10

Try it Yourself »

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Difference Between var, let and const

	Scope	Redeclare	Reassign	Hoisted	Binds this
var	No	Yes	Yes	Yes	Yes
let	Yes	No	Yes	No	No
const	Yes	No	No	No	No

What is Good?

let and const have block scope.

let and const can not be redeclared.

let and const must be declared before use.

let and const does not bind to this.

let and const are not hoisted.

What is Not Good?

var does not have to be declared.

var is hoisted.

var binds to this.