JavaScript Let

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

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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

{

  let x = 2;

  }

// x can NOT be used here

Function Scope

Inside a function all variables declared with var, let or const have Function Scope:

Example

function myfunction() {

  var x = 1;

  let y = 2;

  const z = 3;

}

//x can NOT be used here

//y can NOT be used here

//z 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

{

  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.

Example

let x = "John Doe";

let x = 0;

Variables defined with var can be redeclared.

Example

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

The difference between var, let, and const lies in scoping mechanisms, reassignment rules, and JavaScript hoisting.

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

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";

Video: JavaScript let