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 cannot 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 var inside 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:

let x = "John Doe";

let x = 0;

Variables defined with varcan be redeclared.

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:

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:

let x = 10;
// Here x is 10

{
let x = 2;
// Here x is 2
}

// Here x is 10

Difference Between var, let and const

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.

Browser Support

The let and const keywords are not supported in Internet Explorer 11 or earlier. The following table defines the first browser versions with full support:

Redeclaring

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

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:

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:

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:

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”:

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