code: function callback_(fn, obj) {
    var args = [];
    for (var i = 2, c = arguments.length; i < c; i++) {
      args.push(arguments[i]);
    }
    return function() {
      fn.apply(obj, args);
    };
  }
docs: create a callback closure
  @private
  @param {Object} fn
  @param {Object} obj