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Constants are block-scoped, much like variables defined using the let statement. The value of a constant cannot change through re-assignment, and it can't be redeclared.

Syntax

const name1 = value1 [, name2 = value2 [, ... [, nameN = valueN]]];
nameN
The constant's name, which can be any legal identifier.
valueN
The constant's value; this can be any legal expression, including a function expression.

Description

This declaration creates a constant whose scope can be either global or local to the block in which it is declared. Global constants do not become properties of the window object, unlike var variables. An initializer for a constant is required; that is, you must specify its value in the same statement in which it's declared (which makes sense, given that it can't be changed later).

The const declaration creates a read-only reference to a value. It does not mean the value it holds is immutable, just that the variable identifier cannot be reassigned. For instance, in the case where the content is an object, this means the object's contents (e.g., its parameters) can be altered.

All the considerations about the "temporal dead zone" apply to both let and const.

A constant cannot share its name with a function or a variable in the same scope.

Examples

The following example demonstrates how constants behave. Try this in your browser console.

// NOTE: Constants can be declared with uppercase or lowercase, but a common
// convention is to use all-uppercase letters.

// define MY_FAV as a constant and give it the value 7
const MY_FAV = 7;

// this will throw an error - Uncaught TypeError: Assignment to constant variable.
MY_FAV = 20;

// MY_FAV is 7
console.log('my favorite number is: ' + MY_FAV);

// trying to redeclare a constant throws an error -  Uncaught SyntaxError: Identifier 'MY_FAV' has already been declared
const MY_FAV = 20;

// the name MY_FAV is reserved for constant above, so this will fail too
var MY_FAV = 20;

// this throws an error too
let MY_FAV = 20;

// it's important to note the nature of block scoping
if (MY_FAV === 7) { 
    // this is fine and creates a block scoped MY_FAV variable 
    // (works equally well with let to declare a block scoped non const variable)
    let MY_FAV = 20;

    // MY_FAV is now 20
    console.log('my favorite number is ' + MY_FAV);

    // this gets hoisted into the global context and throws an error
    var MY_FAV = 20;
}

// MY_FAV is still 7
console.log('my favorite number is ' + MY_FAV);

// throws an error - Uncaught SyntaxError: Missing initializer in const declaration
const FOO; 

// const also works on objects
const MY_OBJECT = {'key': 'value'};

// Attempting to overwrite the object throws an error - Uncaught TypeError: Assignment to constant variable.
MY_OBJECT = {'OTHER_KEY': 'value'};

// However, object keys are not protected,
// so the following statement is executed without problem
MY_OBJECT.key = 'otherValue'; // Use Object.freeze() to make object immutable

// The same applies to arrays
const MY_ARRAY = [];
// It's possible to push items into the array
MY_ARRAY.push('A'); // ["A"]
// However, assigning a new array to the variable throws an error - Uncaught TypeError: Assignment to constant variable.
MY_ARRAY = ['B'];

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Let and Const Declarations' in that specification.
Standard Initial definition.
ECMAScript Latest Draft (ECMA-262)
The definition of 'Let and Const Declarations' in that specification.
Living Standard No changes.

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support21 Yes361 211 Yes5.1
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic support Yes Yes Yes361 211 Yes Yes

1. Prior to Firefox 13, const is implemented, but re-assignment is not failing.

2. Prior to Firefox 46, a TypeError was thrown on redeclaration instead of a SyntaxError.

See also