Even smaller Object extend() method, with no reference cloning: http://jsfiddle.net/LR639

extend.js

function extend (target, source) {
    var a = Object.create(target);
    Object.keys(source).map(function (prop) {
        prop in a && (a[prop] = source[prop]);
    });
    return a;
};

In your example the target is set as your objects prototype possible duplicates set as properties.

The prototype accessible one has the original value whilst the property one has the new value. This is according the Chrome and IE's dev tools however.

Was this intended?

See my updated fiddle it explains it better,
http://jsfiddle.net/LR639/1/

Also i sounded quite arrogant there sorry, tried to change my comment but GitHub keeps throwing an error when i try :)

Hi @toddmotto! What you think about this?

function extend(target, source) {
  return Object.assign({}, defaults, options);
};