View Categories

JavaScript Let

3 min read

ECMAScript 2015 #

ES2015 introduced two important new JavaScript keywords: let and const.

These two keywords provide Block Scope variables (and constants) in JavaScript.

Before ES2015, JavaScript had only two types of scope: Global Scope and Function Scope


Global Scope #

Variables declared Globally (outside any function) have Global Scope.

Example #

var carName = "Volvo"; // code here can use carName function myFunction() { // code here can also use carName }

Global variables can be accessed from anywhere in a JavaScript program.


Function Scope #

Variables declared Locally (inside a function) have Function Scope.

Example #

// code here can NOT use carName function myFunction() { var carName = "Volvo"; // code here CAN use carName } // code here can NOT use carName

Local variables can only be accessed from inside the function where they are declared.


JavaScript Block Scope #

Variables declared with the var keyword cannot have Block Scope.

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

Example #

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

Before ES2015 JavaScript did not have Block Scope.

Variables declared with the let keyword can have Block Scope.

Variables declared inside a block {} cannot be accessed from outside the block:

Example #

{ let x = 2; } // x can NOT 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 #

var 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

Loop Scope #

Using var in a loop:

Example #

var i = 5; for (var i = 0; i < 10; i++) { // some statements } // Here i is 10

Using let in a loop:

Example #

let i = 5; for (let i = 0; i < 10; i++) { // some statements } // Here i is 5

In the first example, using var, the variable declared in the loop redeclares the variable outside the loop.

In the second example, using let, the variable declared in the loop does not redeclare the variable outside the loop.

When let is used to declare the i variable in a loop, the i variable will only be visible within the loop.


 

Function Scope #

Variables declared with var and let are quite similar when declared inside a function.

They will both have Function Scope:

 
function myFunction() { var carName = "Volvo"; // Function Scope } function myFunction() { let carName = "Volvo"; // Function Scope }

Global Scope #

Variables declared with var and let are quite similar when declared outside a block.

They will both have Global Scope:

 
var x = 2; // Global scope let x = 2; // Global scope

Global Variables in HTML #

With JavaScript, the global scope is the JavaScript environment.

In HTML, the global scope is the window object.

Global variables defined with the var keyword belong to the window object:

Example #

var carName = "Volvo"; // code here can use window.carName

Global variables defined with the let keyword do not belong to the window object:

Example #

let carName = "Volvo"; // code here cannot use window.carName

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

Redeclaring a var variable with let, in the same scope, or in the same block, is not allowed:

Example #

var x = 2; // Allowed let x = 3; // Not allowed { var x = 4; // Allowed let x = 5 // Not allowed }

Redeclaring a let variable with let, in the same scope, or in the same block, is not allowed:

Example #

let x = 2; // Allowed let x = 3; // Not allowed { let x = 4; // Allowed let x = 5; // Not allowed }

Redeclaring a let variable with var, in the same scope, or in the same block, is not allowed:

Example #

let x = 2; // Allowed var x = 3; // Not allowed { let x = 4; // Allowed var x = 5; // Not allowed }

Redeclaring a variable with let, in another scope, or in another block, is allowed:

Example #

let x = 2; // Allowed { let x = 3; // Allowed } { let x = 4; // Allowed }

Hoisting #

Variables defined with var are hoisted to the top and can be initialized at any time (if you don’t know what Hoisting is, read our Hoisting Chapter).

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

Example #

This is OK:

carName = "Volvo"; alert(carName); var carName;

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

Meaning: The block of code is aware of the variable, but it cannot be used until it has been declared.

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

The variable is in a “temporal dead zone” from the start of the block until it is declared:

Example #

This will result in a ReferenceError:

carName = "Volvo"; let carName;

Powered by BetterDocs

Leave a Reply