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

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"; 

Answer 2

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.