How do you implement an extend function that takes an object and extends itwith new properties and makes it work on n levels of recursion? Basically, duplicating a jQuery extend.

gistfile1.js

/**
 * Implement Foo Object.
 * - Take a JSON object as an input parameter
 * - Create a copy this JSON object as itself.
 * - Chainable
 */
var foo = new Foo({
  version : '1.0'
});

// add components
foo
  .set({
    type : 'Foo',
    container : '#foo'
  })
  // add features
  .addItem(['item1','item3'])
  .addItem('item2');
  
/**
 * Expect output
 */ 
foo = {
  version : '1.0',
  type : 'Foo',
  container : '#foo',
  items : ['item1', 'item3', 'item2']
}

gistfile2.js

/**
 * Foo Object
 */
var Foo = function (config) {
  var options = JSON.parse( JSON.stringify(config) );
  options.items = [];
  _.extend(this, options);
};

Foo.prototype.set = function (config) {
  _.extend(this, config);
  return this;
};

Foo.prototype.addItem = function (items) {
  items = _(items).isArray() ? items : [items];
  this.items = _.union(this.items, items);
  return this;
};