Are there constants in Javascript?

Question

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

Answer 1

No, not in general. FireFox implements const but I know IE doesn't.

---

@John points to a common naming practice for consts that has been used for years in other languages, I see no reason why you couldn't use that. Of course that doesn't mean someone will not write over the variable's value anyway. :)

Answer 2

var MY_CONSTANT = "some-value";

Nothing's ever constant, but you can use conventions like ALL_CAPS to show that certain values should not be modified.

Answer 3

I use const instead of var in my Greasemonkey scripts, but it is because they will run only on Firefox...
Name convention can be indeed the way to go, too (I do both!).

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

Are you trying to protect the variables against modification? If so, then you can use a module pattern:

var CONFIG = (function() {
     var private = {
         'MY_CONST': '1',
         'ANOTHER_CONST': '2'
     };

     return {
        get: function(name) { return private[name]; }
    };
})();

alert('MY_CONST: ' + CONFIG.get('MY_CONST'));  // 1

CONFIG.MY_CONST = '2';
alert('MY_CONST: ' + CONFIG.get('MY_CONST'));  // 1

CONFIG.private.MY_CONST = '2';                 // error
alert('MY_CONST: ' + CONFIG.get('MY_CONST'));  // 1

Using this approach, the values cannot be modified. But, you have to use the get() method on CONFIG :(.

If you don't need to strictly protect the variables value, then just do as suggested and use a convention of ALL CAPS.

Answer 6

There is the const keyword in JavaScript, but it thus far only enjoys a smattering of browser support (see the "Watch It!" sidebar).

Answer 7

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>