实现简单的Promise

class Promise{
  constructor(executor){
    // 默认是等待状态
    this.status = 'pending';
    this.value = undefined;
    this.reason = undefined;
    // 存放成功回调函数
    this.onResolveCallbacks = [];
    this.onRejectedCallbacks = [];
    let resolve = (data) => {
      // this指的是实例
      if(this.status === 'pending'){
        this.value = data;
        this.status = 'resolved';
        this.onResolveCallbacks.forEach(fn=>fn());
      }
    }
    let reject = (reason) => {
      // this指的是实例
      if(this.status === 'pending'){
        this.value = reason;
        this.status = 'rejected';
        this.onRejectedCallbacks.forEach(fn=>fn());
      }
    }
    // 执行时可能发生异常
    try{ 
      executor(resolve,reject);
    }catch(e){
      reject(e);
    }
  }
  then(onFulfilled,onRejected){
    if(this.status === 'resolve'){
      onFulfilled(this.value);
    }
    if(this.status === 'reject'){
      onRejected(this.value);
    }
    if(this.status === 'pending'){
      this.onReslovedCallbacks.push(()=>{
        onFulfilled(this.value);
      })
      this.onRejectedCallbacks.push(()=>{
        onRejected(this.reason);
      })
    }
  }
}

参考文章：Promise不会？？看这里！！！史上最通俗易懂的Promise！！！