JavaScript Let Keyword

 

The let keyword was introduced in ES6 (2015)

Variables declared with let have Block Scope

Variables declared with let must be Declared before use

Variables declared with let cannot be Redeclared in the same scope

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

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

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;

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

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

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.

Redeclaring

Redeclaring a JavaScript variable with var is allowed anywhere in a program:

Example

var x = 2;
// Now x is 2

var x = 3;
// Now x is 3

With let, redeclaring a variable in the same block is NOT allowed:

Example

var x = 2;   // Allowed
let x = 3;   // Not allowed

{
let x = 2;   // Allowed
let x = 3;   // Not allowed
}

{
let x = 2;   // Allowed
var x = 3;   // Not allowed
}

Redeclaring a variable with let, in another block, IS allowed:

Example

let x = 2;   // Allowed

{
let x = 3;   // Allowed
}

{
let x = 4;    // Allowed
}

Let Hoisting

Variables defined with var are hoisted to the top and can be initialized at any time.

Meaning: You can use the variable before it is declared:

Example

This is OK:

carName = "Volvo";
var carName;

If you want to learn more about hoisting, study the chapter JavaScript Hoisting.

Variables defined with let are also hoisted to the top of the block, but not initialized.

Meaning: Using a let variable before it is declared will result in a ReferenceError:

Example

carName = "Saab";
let carName = "Volvo";