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;
};

Hi @toddmotto! What you think about this?

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