óCoffeeScript Cookbook

Extending Built-in Objects


You want to extend a class to add new functionality or replace old.


Use :: to assign your new function to the prototype of the object or class.

String::capitalize = () ->
  (this.split(/\s+/).map (word) -> word[0].toUpperCase() + word[1..-1].toLowerCase()).join ' '

"foo bar     baz".capitalize()
# => 'Foo Bar Baz'


Objects in JavaScript (and thus, in CoffeeScript) have a prototype member that defines what member functions should be available on all objects based on that prototype. In CoffeeScript, you can access the prototype directly via the :: operator.

Note: Although it's quite common in languages like Ruby, extending native objects is often considered bad practice in JavaScript (see: Maintainable JavaScript: Don’t modify objects you don’t own; Extending built-in native objects. Evil or not?).