const

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.

SyntaxEdit

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.

DescriptionEdit

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.

ExamplesEdit

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 fail silently 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; 

// 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;

// const requires an initializer
const FOO; // SyntaxError: missing = in const declaration

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

// Overwriting the object fails as above (in Firefox and Chrome but not in Safari)
MY_OBJECT = {"OTHER_KEY": "value"};

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

SpecificationsEdit

Browser compatibilityEdit

Compatibility notesEdit

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 alsoEdit

• var
• let
• Constants in the JavaScript Guide