gistfile1.js

Array.prototype.contains = function(item) {
   return this.indexOf(item) != -1;
};

var a = ['foo', 'bar', 'baz'];
alert(a.contains('bar')); 


// Handy but arguably not always a good idea...
// String.prototype  and Array.prototype are themselves global namespaces, and adding properties to them brings the same risk of collisions as being careless with regular globals...
//  -- http://simonwillison.net/2007/Aug/15/jquery/