JavaScript Let

 

 

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;

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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

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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

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 

 

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
}

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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 HTML initialized.

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

Example

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