code: function mirrorProperties(src, dest = {}) {
    function set(name) { dest[name] = src[name]; }
    function _delete(name) { delete dest[name]; }
  
    var handlers = { add: set, update: set, delete: _delete};
  
    assign(dest, src);
    observe(src, makeObserver(handlers));
    return dest;
  }
docs: Keep an object in sync with another.