源碼看這里Promise.prototype.then = function(onFulfilled, onRejected) {  if (this.constructor !== Promise) {    return safeThen(this, onFulfilled, onRejected);  }  var res = new Promise(noop);  handle(this, new Handler(onFulfilled, onRejected, res));  return res;};function safeThen(self, onFulfilled, onRejected) {  return new self.constructor(function (resolve, reject) {    // 以下不是很理解    var res = new Promise(noop);    res.then(resolve, reject);    handle(self, new Handler(onFulfilled, onRejected, res));  });}