实现一个Promise之基础、异步

//promise三个状态
const PENDING = 'PENDING';
const FULFILLED = 'FULFILLED';
const REJECTED = 'REJECTED';
class Promise {
 constructor(executor){
  //状态
  this.state = PENDING;
  //成功值 失败值 默认undefined
  this.value = undefined;
  this.reason = undefined;
  //简易发布订阅，实例可以then多次，用数组
  this.onResolvedCallbacks = [];
  this.onRejectedCallbacks = [];
  //成功方法
  let resolve = (value) => {
   if(this.state === PENDING){
    this.state = FULFILLED;
    this.value = value;
    this.onResolvedCallbacks.forEach(fn => fn());
   }
  };
  //失败方法
  let reject = (reason) => {
   if(this.state === PENDING){
    this.state = REJECTED;
    this.reason = reason;
    this.onRejectedCallbacks.forEach(fn => fn());
   }
  };
  //立即执行，如果有错误直接执行失败方法
  try{
   executor(resolve, reject);
  }catch (e) {
   reject(e);
  }
 }
 //then方法，一个成功函数，一个失败函数
 then(onFulfilled, onRejected){
  //如果成功，执行成功函数
  if(this.state === FULFILLED){
   onFulfilled(this.value);
  };
  //如果失败，执行失败函数
  if(this.state === REJECTED){
   onRejected(this.reason);
  };
  //如果有异步，订阅成功失败函数
  if(this.state === PENDING){
   this.onResolvedCallbacks.push(() => {
    onFulfilled(this.value);
   });
   this.onRejectedCallbacks.push(() => {
    onRejected(this.reason);
   });
  };
 }
}