Are There Constants in JavaScript

Are there constants in JavaScript?

Since ES2015, JavaScript has a notion of const:

const MY_CONSTANT = "some-value";

This will work in pretty much all browsers except IE 8, 9 and 10. Some may also need strict mode enabled.

You can use var with conventions like ALL_CAPS to show that certain values should not be modified if you need to support older browsers or are working with legacy code:

var MY_CONSTANT = "some-value";

Can you use constant variables in JavaScript?

const is a proposed feature of ECMAScript Harmony (together with a properly block-scoped let it is supposed to replace var and implicit globals). ECMAScript Harmony is a grab-bag of ideas for the next versions of ECMAScript.

const was also a part of ECMAScript 4.

ECMAScript 4 was never released and never will be, and ECMAScript Harmony will only be released in a couple of years. Therefore, you cannot reliably use it.

There are some implementations or derivatives of ECMAScript that implement const (ActionScript, for example). There are also some implementations that accept const as a synonym for var (IOW, you can use const, but it won't give you any protection.)

However, unless you absolutely can guarantee that your code will only run on very specific versions of very specific implementations of very specific derivatives of ECMAScript, it's probably better to avoid it. (Which is a real shame, because const and especially let are a huge improvement over var and implicit globals.)

Where are JavaScript constants stored in browser?

Only global variables get assigned a matching property on the global object. Nothing else, including constants, does.

The value of a constant won't automatically appear anywhere except in the constant itself.

If you want to access it via a string, then store it as a property of an object (preferably not the global object though, that's a disorganised mess with a significant risk of clashing with other code).

Can global constants be declared in JavaScript?

Javascript doesn't really have the notion of a named constant, or an immutable property of an object. (Note that I'm not talking about ES5 here.)

You can declare globals with a simple var declaration in the global scope, like outside any function in a script included by a web page:

<script>
  var EXACTLY_ONE = 1;

Then your code can use that constant of course, though it's not really "constant" because the value can be changed (the property updated, in other words).

edit — this is an ancient answer to an ancient question. In 2019, there's the const declaration that's supported just about everywhere. However note that like let, const scoping is different from var scoping.

Why constants' property or value aren't constant

What does const mean?

If a variable is declared using const keyword it means it can't be assigned to new value. It doesnot mean that you can't modify it. Changing the properties of object is modifying.

Solution

If you want an object not to change you can use Object.freeze()

let someObj = {};
someObj.prop = 1; //setting prop to '1'

Object.freeze(someObj); //after this line you can't modify object
someObj.prop = "changed"; //prop will remain '1'
console.log(someObj)

JavaScript constants | Style and keyword

Update 2017

ES6, or ECMAScript2017, comes with the support for constants using the keyword const.

While some use camelCase syntax for the constant's names, it still is perfectly valid to use the capitalized (snake_case) version (e.g. const VARIABLE_NAME;)

---

Original answer

JavaScript does not have constants.

You could use the convention of all uppercase for constants.

var PI = 3.14;

or you could wrap it in an object.

window.CONSTANTS = {
   PI : 3.14
};

Const in JavaScript: when to use it and is it necessary?

There are two aspects to your questions: what are the technical aspects of using const instead of var and what are the human-related aspects of doing so.

The technical difference is significant. In compiled languages, a constant will be replaced at compile-time and its use will allow for other optimizations like dead code removal to further increase the runtime efficiency of the code. Recent (loosely used term) JavaScript engines actually compile JS code to get better performance, so using the const keyword would inform them that the optimizations described above are possible and should be done. This results in better performance.

The human-related aspect is about the semantics of the keyword. A variable is a data structure that contains information that is expected to change. A constant is a data structure that contains information that will never change. If there is room for error, var should always be used. However, not all information that never changes in the lifetime of a program needs to be declared with const. If under different circumstances the information should change, use var to indicate that, even if the actual change doesn't appear in your code.

---

Related Topics