实现一个promise之完成then方法

最简单的promise完成了，接着就要完善then方法：

//...//解决返回结果function resolvePromise(promise2, x, resolve, reject) {};class Promise {//... //then方法，一个成功函数，一个失败函数 then(onFulfilled, onRejected){  //假如传进来的不是函数（不传也算），要忽略，赋值一个函数  // 错误函数要抛出错误才能进入reject  onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : y => y;  onRejected = typeof onRejected === 'function' ? onRejected : r => {throw r};  //then必须返回一个promise,由于要传递成功失败结果  // 给下一个then，所以把方法都放入这个方法  let promise2 = new Promise((resolve, reject) => {   //假如成功，执行成功函数   if(this.state === FULFILLED){    /*成功或者者失败函数返回一个值x，假如报错直接reject     每个人写的promise不同，但是要兼容符合规范的promise     要对x进行兼容解决，返回普通值、方法、对象，其它人的promise实例等     这边要先使用微任务MutationObserver、process.nextTick     假如不兼容微任务才使用宏任务setTimeout、setImmediate     这边直接用setTimeout    */    setTimeout(() => {     try{      let x = onFulfilled(this.value);      resolvePromise(promise2, x, resolve, reject)     }catch (e) {      reject(e);     };    }, 0);   };      //假如失败，执行失败函数   if(this.state === REJECTED){    setTimeout(() => {     try{      let x = onRejected(this.reason);      resolvePromise(promise2, x, resolve, reject)     }catch (e) {      reject(e);     };    }, 0);   };      //假如有异步，订阅成功、失败函数   if(this.state === PENDING){    this.onResolvedCallbacks.push(() => {     setTimeout(() => {      try{       let x = onFulfilled(this.value);       resolvePromise(promise2, x, resolve, reject)      }catch (e) {       reject(e);      };     }, 0);    });        this.onRejectedCallbacks.push(() => {     setTimeout(() => {      try{       let x = onRejected(this.reason);       resolvePromise(promise2, x, resolve, reject)      }catch (e) {       reject(e);      };     }, 0);    });   };  });  //返回promise  return promise2; }}