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JavaScript Const

The const keyword was introduced in ES6 (2015).

Variables defined with const cannot be Redeclared.

Variables defined with const cannot be Reassigned.

Variables defined with const have Block Scope.

Cannot be Reassigned

A const variable cannot be reassigned:


const PI = 3.141592653589793;
PI = 3.14;      // This will give an error
PI = PI + 10;   // This will also give an error
Try it Yourself »

Must be Assigned

JavaScript const variables must be assigned a value when they are declared:


const PI = 3.14159265359;


const PI;
PI = 3.14159265359;

When to use JavaScript const?

Always declare a variable with const when you know that the value should not be changed.

Use const when you declare:

  • A new Array
  • A new Object
  • A new Function
  • A new RegExp

Constant Objects and Arrays

The keyword const is a little misleading.

It does not define a constant value. It defines a constant reference to a value.

Because of this you can NOT:

  • Reassign a constant value
  • Reassign a constant array
  • Reassign a constant object

    But you CAN:

  • Change the elements of constant array
  • Change the properties of constant object

Constant Arrays

You can change the elements of a constant array:


// You can create a constant array:
const cars = ["Saab", "Volvo", "BMW"];

// You can change an element:
cars[0] = "Toyota";

// You can add an element:
Try it Yourself »

But you can NOT reassign the array:


const cars = ["Saab", "Volvo", "BMW"];

cars = ["Toyota", "Volvo", "Audi"];    // ERROR
Try it Yourself »

Constant Objects

You can change the properties of a constant object:


// You can create a const object:
const car = {type:"Fiat", model:"500", color:"white"};

// You can change a property:
car.color = "red";

// You can add a property:
car.owner = "Johnson";
Try it Yourself »

But you can NOT reassign the object:


const car = {type:"Fiat", model:"500", color:"white"};

car = {type:"Volvo", model:"EX60", color:"red"};    // ERROR
Try it Yourself »

Browser Support

The const keyword is not fully supported in Internet Explorer.

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

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

Block Scope

Declaring a variable with const is similar to let when it comes to Block Scope.

The x declared in the block, in this example, is not the same as the x declared outside the block:


const x = 10;
// Here x is 10

const x = 2;
// Here x is 2

// Here x is 10
Try it Yourself »

You can learn more about block scope in the chapter JavaScript Scope.


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


var x = 2;     // Allowed
var x = 3;     // Allowed
x = 4;         // Allowed

Redeclaring an existing var or let variable to const, in the same scope, is not allowed:


var x = 2;     // Allowed
const x = 2;   // Not allowed

let x = 2;     // Allowed
const x = 2;   // Not allowed

const x = 2;   // Allowed
const x = 2;   // Not allowed

Reassigning an existing const variable, in the same scope, is not allowed:


const x = 2;     // Allowed
x = 2;           // Not allowed
var x = 2;       // Not allowed
let x = 2;       // Not allowed
const x = 2;     // Not allowed

  const x = 2;   // Allowed
  x = 2;         // Not allowed
  var x = 2;     // Not allowed
  let x = 2;     // Not allowed
  const x = 2;   // Not allowed

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


const x = 2;       // Allowed

  const x = 3;   // Allowed

  const x = 4;   // Allowed


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:


This is OK:

carName = "Volvo";
var carName;
Try it Yourself »

If you want to learn more about hoisting, study the chapter JavaScript Hoisting.

Variables defined with const are also hoisted to the top, but not initialized.

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


alert (carName);
const carName = "Volvo";
Try it Yourself »