-- Main.shssdhakchina - 26 Jan 2022

let and const keywords

• The let keyword was introduced in ES6 (2015).
• Variables defined with let cannot be Redeclared.
• Variables defined with let must be Declared before use.
• Variables defined with let have Block Scope.

Cannot be Redeclared

Variables defined with let cannot be redeclared.

You cannot accidentally redeclare a variable.

With let you can not do this:

Example

let x = "John Doe";
let x = 0;
// SyntaxError: 'x' has already been declared

With var you can:

Example

var x = "John Doe";
var x = 0;

Block Scope

Before ES6 (2015), JavaScript had only Global Scope and Function Scope.

ES6 introduced two important new JavaScript keywords: let and const. These two keywords provide Block Scope in JavaScript. Variables declared inside a { } block cannot be accessed from outside the block:

Example

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

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

Variables declared inside a { } block can be accessed from outside the block.

Example

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

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

Browser Support

The let keyword is not fully supported in Internet Explorer 11 or earlier.

The following table defines the first browser versions with full support for the let keyword:

Chrome 49 Edge 12 Firefox 44 Safari 11 Opera 36 Mar, 2016 Jul, 2015 Jan, 2015 Sep, 2017 Mar, 2016

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
}