Are there constants in JavaScript?

Question

Is there a way to use constants in JavaScript?

If not, what\'s the common practice for specifying variables that are used as constants?

Answer 1

ECMAScript 5 does introduce Object.defineProperty:

Object.defineProperty (window,'CONSTANT',{ value : 5, writable: false });

It's supported in every modern browser (as well as IE ≥ 9).

See also: Object.defineProperty in ES5?

Answer 2

In JavaScript, my preference is to use functions to return constant values.

function MY_CONSTANT() {
   return "some-value";
}


alert(MY_CONSTANT());

Answer 3

Rhino.js implements const in addition to what was mentioned above.

Answer 4

For a while, I specified "constants" (which still weren't actually constants) in object literals passed through to with() statements. I thought it was so clever. Here's an example:

with ({
    MY_CONST : 'some really important value'
}) {
    alert(MY_CONST);
}

In the past, I also have created a CONST namespace where I would put all of my constants. Again, with the overhead. Sheesh.

Now, I just do var MY_CONST = 'whatever'; to KISS.

Answer 5

The const keyword is in the ECMAScript 6 draft but it thus far only enjoys a smattering of browser support: http://kangax.github.io/compat-table/es6/. The syntax is:

const CONSTANT_NAME = 0;

Answer 6

IE does support constants, sort of, e.g.:

<script language="VBScript">
 Const IE_CONST = True
</script>
<script type="text/javascript">
 if (typeof TEST_CONST == 'undefined') {
    const IE_CONST = false;
 }
 alert(IE_CONST);
</script>