Function.prototype.extend = function(fn) {

  var self = this;

  return function(){

    self.apply(this, arguments);

    fn.apply(this, arguments);

  };

};

​

window.onload = function() {

  console.log('foo');

};

​

window.onload = window.onload.extend(function() {

  console.log('bar');

});

​

// Logs "foo" and "bar"