Non-Instance Safe JS

instance-pollution.js

function Cat(colour, size) {
  this.colour = colour;
  this.size = size;

}

Cat.prototype.meow = function() {
  console.log("Meow");
};

function Garfield() {
}

Garfield.prototype = new Cat('orange', 'fat');

var gf = new Garfield();

gf.meow(); //Meow, as expected.

Cat.prototype.swish = function() {
  console.log("TAIL SWISH!");
};

gf.swish(); //TAIL SWISH, even though it wasn't part of the original inheritance. 

@VengadoraVG: Thanks for the comment, however this particular Gist was used as part of a blog post to demonstrate precisely Elliot's technique. Please see: http://www.donburks.com/mixin-up-your-objects/