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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:

Example

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:

Correct

const PI = 3.14159265359;

Incorrect

const PI;
PI = 3.14159265359;

When to use JavaScript const?

As a general rule, always declare a variables with const unless you know that the value will change.

Always 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 a constant array
  • Change a constant object

Constant Arrays

You can change the elements of a constant array:

Example

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

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

// You can add an element:
cars.push("Audi");
Try it Yourself »

But you can NOT reassign the array:

Example

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

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

Constant Objects

You can change the properties of a constant object:

Example

// 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:

Example

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 supported in Internet Explorer 10 or earlier.

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

Chrome 49 IE 11 / Edge Firefox 36 Safari 10 Opera 36
Mar, 2016 Oct, 2013 Feb, 2015 Sep, 2016 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:

Example

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

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

Example

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:

Example

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:

Example

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:

Example

const x = 2;       // Allowed

{
  const x = 3;   // Allowed
}

{
  const x = 4;   // Allowed
}

Const 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;
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:

Example

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