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";

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;

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.

"use strict";

var constants = Object.freeze({
    "π": 3.141592653589793 ,
    "e": 2.718281828459045 ,
    "i": Math.sqrt(-1)
});

constants.π;        // -> 3.141592653589793
constants.π = 3;    // -> TypeError: Cannot assign to read only property 'π' …
constants.π;        // -> 3.141592653589793

delete constants.π; // -> TypeError: Unable to delete property.
constants.π;        // -> 3.141592653589793

See Object.freeze. You can use const if you want to make the constants reference read-only as well.