ECMAScript 5 extend(), preserves original Object values

extend.js

function extend (target, source) {
    target = JSON.parse(JSON.stringify(target));
    Object.keys(source).map(function (prop) {
        Object.prototype.hasOwnProperty.call(target, prop) && (target[prop] = source[prop]);
    });
    return target;
};

Demo: http://jsfiddle.net/7d734