I want to add the ability to extend javascript objects by adding a method to the prototype.
The method will receive one or more other objects and will add all of the key/values to this.
This is what I came up with:

Object::extend = (objects...) ->
    @[key] = value for key, value of object for object in objects

or this:

Object::extend = (objects...) ->
    for object in objects
        for key, value of object
            @[key] = value

Both work as expected, and compile into the same javascript code:

var __slice = [].slice;

Object.prototype.extend = function() {
  var key, object, objects, value, _i, _len, _results;
  objects = 1 <= arguments.length ? __slice.call(arguments, 0) : [];
  _results = [];
  for (_i = 0, _len = objects.length; _i < _len; _i++) {
    object = objects[_i];
    _results.push((function() {
      var _results1;
      _results1 = [];
      for (key in object) {
        value = object[key];
        _results1.push(this[key] = value);
      }
      return _results1;
    }).call(this));
  }
  return _results;
};

What I'm not too happy about is the whole results thing that is created per for loop which is completely redundant for my purpose.
Is there a way to get a code more like:

Object.prototype.extend = function() {
  var key, object, objects, value, _i, _len;
  objects = 1 <= arguments.length ? __slice.call(arguments, 0) : [];
  for (_i = 0, _len = objects.length; _i < _len; _i++) {
    object = objects[_i];
    (function() {
      for (key in object) {
        value = object[key];
        this[key] = value;
      }
    }).call(this);
  }
};

Thanks.

Edit

I'm aware that I can simply embed javascript code, but looking for a coffeescript solution.

You can try adding an explicit return:

Object::extend = (objects...) ->
    for object in objects
        for key, value of object
            @[key] = value
    return

That produces this:

var __slice = [].slice;

Object.prototype.extend = function() {
  var key, object, objects, value, _i, _len;
  objects = 1 <= arguments.length ? __slice.call(arguments, 0) : [];
  for (_i = 0, _len = objects.length; _i < _len; _i++) {
    object = objects[_i];
    for (key in object) {
      value = object[key];
      this[key] = value;
    }
  }
};

Every CoffeeScript function returns the value of the last expression in the function, CoffeeScript loops are also expressions. That means that CoffeeScript has to build all that _results stuff to produce a return value for your function since you had an implicit return applying to the outer loop. If you remove that implicit return by adding an explicit "return nothing" then the CS→JS compiler seems to be smart enough to not do all that extra _results work.