/**
 * Array.prototype.[method name] allows you to define/overwrite an objects method
 * needle is the item you are searching for
 * this is a special variable that refers to "this" instance of an Array.
 * returns true if needle is in the array, and false otherwise
 */
Array.prototype.contains = function ( needle ) {
  for (i in this) {
    if (this[i] == needle) return true;
  }
  return false;
}

// Now you can do things like:
var x = Array();
if (x.contains('foo')) {
  // do something special
}