实现一个promise之完成then方法

//...
//处理返回结果
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){
  //如果传进来的不是函数（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必需返回一个promise,因为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) => {
   //如果成功，执行成功函数
   if(this.state === FULFILLED){
    /*成功或者失败函数返回一个值x，如果报错直接reject
     每个人写的promise不同，但是要兼容符合规范的promise
     要对x进行兼容处理，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
     如果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);
   };
   //如果失败，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);
   };
   //如果有异步，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;
 }
}