SOURCE

class MyPromise{
  constructor(executor){
      this.initValue()
      this.initBind()
      try {
      executor(this.resolve,this.reject)
      }catch(e){
          this.reject(e)
      }
  }
  initValue(){
      this.PromiseState='pending';
      this.PromiseResult=null;
      //因为一个promise实例可能会多次then，用数组就一个一个保存了
      this.onFulfilledCallbacks=[] //保存成功回调
      this.onRejectedCallbacks=[] //保存失败回调
  }
  initBind(){
      this.resolve=this.resolve.bind(this);
      this.reject=this.reject.bind(this);
  }
  resolve(value){
      if(this.PromiseState!=='pending') return ;
      this.PromiseState='fulfilled';
      this.PromiseResult=value;
      //执行保存的成功回调
      while(this.onFulfilledCallbacks.length){
          this.onFulfilledCallbacks.shift()(this.PromiseResult)
      }
  }
  reject(reason){
      if(this.PromiseState!=='pending') return;
      this.PromiseState='rejected';
      this.PromiseResult=reason;
      //执行保存的失败回调
      while(this.onRejectedCallbacks.length){
          this.onRejectedCallbacks.shift()(this.PromiseResult)
      }
  }
  then(onFulfilled,onRejected){//then函数会在调用promise的时候就执行，不会延时。因此这种情况需要先保存
      //参数校验，确保一定是函数。不是函数时,会读到异常，此时执行reject
      onFulfilled=typeof onFulfilled==='function'? onFulfilled:value=>value
      //不是函数时，返回失败原因
      onRejected=typeof onRejected==='function'?onRejected:reason=>{throw reason}
      if(this.PromiseState==='fulfilled'){//如果当前为成功状态
          //成功了就把成功的结果传给成功回调函数onFulfilled
          onFulfilled(this.PromiseResult) //执行第一个回调
      }else if(this.PromiseState==='rejected'){//如果当前为失败状态
          onRejected(this.PromiseResult)//执行第二个回调
      }else if(this.PromiseState==='pending'){//如果当前还未执行resolve或reject，则需要保存
          this.onFulfilledCallbacks.push(onFulfilled.bind(this))
          this.onRejectedCallbacks.push(onRejected.bind(this))
      }
  }
}


let test1=new MyPromise((resolve,reject)=>{
   setTimeout(() => {
        resolve('成功') // 1秒后输出 成功
        // resolve('成功') // 1秒后输出 失败
    }, 1000)
}).then(res=>console.log(res),err=>console.log(err))