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.

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.

Description

This declaration creates a constant that can be either global or local to the function in which it is declared. 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).

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.

All the considerations about the "temporal dead zone" that apply to let, also apply to 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 in Firefox and Chrome (but does not fail in Safari)
MY_FAV = 20;

// will print 7
console.log("my favorite number is: " + MY_FAV);

// trying to redeclare a constant throws an error
const MY_FAV = 20;

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

// this throws an error also
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)
    const 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);

// Assigning to A const variable is a syntax error
const A = 1; A = 2;

// throws an error, missing initializer in const declaration
const FOO; 

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

// Overwriting the object behaves as above (throws an error in Firefox and Chrome but does not fail in Safari)
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

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Let and Const Declarations' in that specification.
Standard Initial definition.
ECMAScript 2017 Draft (ECMA-262)
The definition of 'Let and Const Declarations' in that specification.
Draft  

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support 21 (Yes) 36 (36) 11 12 5.1
Reassignment fails 20 (Yes) 13 (13) 11 ? ?
Allowed in sloppy mode 49.0          
Feature Android Android Webview Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile Chrome for Android
Basic support No support (Yes) ? ? (Yes) ? (Yes)
Reassignment fails No support (Yes) ? ? (Yes) ? (Yes)
Allowed in sloppy mode No support 49.0         49.0

Compatibility notes

In earlier versions of Firefox & Chrome and as of Safari 5.1.7 and Opera 12.00, if you define a variable with const, you can still change its value later. It is not supported in Internet Explorer 6-10, but is included in Internet Explorer 11.

Firefox-specific notes

The const declaration was implemented in Firefox long before const appeared in the ECMAScript 2015 (ES6) specification. For const ES6 compliance see bug 950547 and bug 611388.

  • Prior to SpiderMonkey 46 (Firefox 46 / Thunderbird 46 / SeaMonkey 2.43), a TypeError was thrown on redeclaration instead of a SyntaxError (bug 1198833).
  • Starting with SpiderMonkey 36 (Firefox 36 / Thunderbird 36 / SeaMonkey 2.33):
    • {const a=1};a now throws a ReferenceError and does not return 1 anymore due to block-scoping.
    • const a; now throws a SyntaxError ("missing = in const declaration"): An initializer is required.
    • const a = 1; a = 2; now also throws a SyntaxError ("invalid assignment to const a").

See also