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 {{Glossary("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
// 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 in Firefox and Chrome but does not fail in Safari
MY_ARRAY = ["B"]
Specifications
| Specification | Status | Comment |
|---|---|---|
| {{SpecName('ES6', '#sec-let-and-const-declarations', 'Let and Const Declarations')}} | {{Spec2('ES6')}} | Initial definition. |
| {{SpecName('ESDraft', '#sec-let-and-const-declarations', 'Let and Const Declarations')}} | {{Spec2('ESDraft')}} |
Browser compatibility
{{CompatibilityTable}}
| Feature | Chrome | Edge | Firefox (Gecko) | Internet Explorer | Opera | Safari |
|---|---|---|---|---|---|---|
| Basic support | {{CompatChrome(21)}} | {{CompatVersionUnknown}} | {{CompatGeckoDesktop(36)}} | 11 | 12 | 5.1 |
| Reassignment fails | {{CompatChrome(20)}} | {{CompatVersionUnknown}} | {{CompatGeckoDesktop(13)}} | 11 | {{CompatUnknown}} | {{CompatUnknown}} |
| Allowed in sloppy mode | {{CompatChrome(49.0)}} |
| Feature | Android | Android Webview | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile | Chrome for Android |
|---|---|---|---|---|---|---|---|
| Basic support | {{CompatNo}} | {{CompatVersionUnknown}} | {{CompatUnknown}} | {{CompatUnknown}} | {{CompatVersionUnknown}} | {{CompatUnknown}} | {{CompatVersionUnknown}} |
| Reassignment fails | {{CompatNo}} | {{CompatVersionUnknown}} | {{CompatUnknown}} | {{CompatUnknown}} | {{CompatVersionUnknown}} | {{CompatUnknown}} | {{CompatVersionUnknown}} |
| Allowed in sloppy mode | {{CompatNo}} | {{CompatChrome(49.0)}} | {{CompatChrome(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 {{geckoRelease(46)}}, a {{jsxref("TypeError")}} was thrown on redeclaration instead of a {{jsxref("SyntaxError")}} ({{bug(1198833)}}).
- Starting with SpiderMonkey 36 {{geckoRelease("36")}}:
{const a=1};anow throws aReferenceErrorand does not return1anymore due to block-scoping.const a;now throws aSyntaxError("missing = in const declaration"): An initializer is required.const a = 1; a = 2;now also throws aSyntaxError("invalid assignment to const a").