Polymorphism in Javascript

gistfile1.js

Object.prototype.test = function(){ console.log('an object')};
Array.prototype.test = function(){ console.log('an array')};
Number.prototype.test = function(){ console.log('a number')};

var a = {};
a.test(); // prints "an object"

a = [];
a.test(); // prints "an array"

a = 1;
a.test(); // prints "a number"

a = "";
a.test(); // prints "an object"

String.prototype.test = function(){ console.log('a string')};

a.test(); // prints "a string"