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.

As "Pointy" so carefully notes, ECMAscript has no such feature. However, JavaScript does:

const a = 7;
document.writeln("a is " + a + ".");

Of course, if you're writing code to put on the web to run in web browsers, this might not help you much. :-)

Everything is global unless declared with the var keyword.

There are no constants either. You can simply declare them without the var keyword.

If you want to ensure global scope you can throw it into the window object:

window.GLOBAL_CONSTANT = "value";

You can do this from within any scope. Constants can then be declared inside functions or closures, though I wouldn't recommend that.

If you only care about supporting newer browsers (or are using a transpiler such as Babel to support older browsers) you can do the following:

1. Create a settings.js file with whatever constants you want and export them:

export const FRUIT = "kiwi";
export const VEGETABLE = "carrot";

2. In files that you want to use them you could then import them as follows:

import * as Settings from './settings.js'

3. Then to use the constants do something like this:

console.log("The unchangeable fruit is " + Settings.FRUIT);

This is a much cleaner approach than trying to implement a global constant, especially when you have multiple JavaScript files that you want to use the constants in.

You could do it with getters and setters like so:

Object.defineProperty(window, 'TAU', { 
    get: function(){return Math.PI*2;}
});

If you want a general function to do this:

function define(name, value){
    Object.defineProperty(window, name, { 
       get: function(){return value;},
       set: function(){throw(name+' is a constant and cannot be redeclared.');},
    });
}

// Example use
define('TAU', Math.PI*2);