code: function extend(target, source) {
    var keys = Object.keys(source);
    for(var i = 0, len = keys.length; i < len; i++) {
      var key = keys[i];
      target[key] = source[key];
    }
    return target;
  }
docs: Merges two objects together, overwriting the second's properties onto the
  first.
  
  @param {Object} target
  @param {Object} source
  @return {Object} result The modified `target` parameter