const

The const declaration creates a read-only named constant.

Syntax

const name1 = value1 [, name2 = value2 [, ... [, nameN = valueN]]];

nameN

Constant name. It can be any legal identifier.

valueN

Value of the constant. It can be any legal expression.

Description

This declaration creates a constant that can be global or local to the function in which it is declared. Constants are block-scoped. The value of a constant cannot change through re-assignment, and a constant cannot be re-declared. An initializer for a constant is required. 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.

// 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 myObject = {"key": "value"};

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

// However, object attributes are not protected,
// so the following statement is executed without problems
myObject.key = "otherValue";

Specifications

Browser 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 has been implemented in Firefox long before const appeared in the ECMAScript 6 specification. For const ES6 compliance see bug 950547 and bug 611388.

• Starting with Gecko 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

• var
• let
• Constants in the JavaScript Guide