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";

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.

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;

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?

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. :)

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.

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!).

In JavaScript my practice has been to avoid constants as much as I can and use strings instead. Problems with constants appear when you want to expose your constants to the outside world:

For example one could implement the following Date API:

date.add(5, MyModule.Date.DAY).add(12, MyModule.Date.HOUR)

But it's much shorter and more natural to simply write:

date.add(5, "days").add(12, "hours")

This way "days" and "hours" really act like constants, because you can't change from the outside how many seconds "hours" represents. But it's easy to overwrite MyModule.Date.HOUR.

This kind of approach will also aid in debugging. If Firebug tells you action === 18 it's pretty hard to figure out what it means, but when you see action === "save" then it's immediately clear.

Declare a readonly named constatnt.

Variables declared via const cannot be re-declared or re-assigned.

Constants can be declared with uppercase or lowercase, but a common convention is to use all-uppercase letters.

// const c;
// c = 9;   //intialization and declearation at same place
const c = 9;
// const c = 9;// re-declare and initialization is not possible
console.log(c);//9