I would to extend the functionality of a JavaScript class, for example, to add utility functions to a specific Object.

Object.prototype

The prototype property can be used to add new properties and functions to any object, including built-in objects. For example, you could extend an Array as follows:

//
// extend Array objects, test if value is in array
// e.g.,  [0,1,2].contains(2) == true
//        [0,1,2].contains('spam') == false
Array.prototype.contains = function(obj) {
  for (var i = 0; i < this.length; i++) {
    if (this[i] == obj) {
      return true;
    }
  }
  return false;
}

This will add the contains() function to all Array objects, even those already instantiated. The prototype property can be used to dynamically add new properties to all objects of a specific type, for example:

function Spam(name, eggs) {
  this.name = name;
  this.eggs = eggs;
}
function Foo(eggs) {
  this.eggs = eggs;
}

myspam = new Spam('Brian', 12);
myfoo = new Foo(36);

function eggsByDozen() {
  return this.eggs / 12;
}
Spam.prototype.dozens = eggsByDozen;
Foo.prototype.dozens = eggsByDozen;

myspam.dozens() // returns 1
myfoo.dozens() // returns 3

In this case, an existing function eggsByDozen() is applied to multiple classes, new objects and even previously instantiated objects of those types will have the new method.