promise

class myPromise {
  constructor(executor) {
    //1.参数的校验
    if (typeof executor !== 'function') {
      throw new TypeError('is not a  function')
    }
    this.initValue()
    this.initBind()
    try {
      executor(this.resolve, this.reject)
    } catch (error) {
      this.reject(error)
    }
  }
    initValue() {
      // 2.初始化数据
      this.value = null;
      this.state = 'pending';
      this.reason = null;
      this.onFulfilledCallBacks = []
      this.onRejectedCallBacks = []
    }
    initBind(){
      this.resolve=this.resolve.bind(this)
      this.reject=this.reject.bind(this)
    }
    resolve(value) {
      // 1.更改promise的状态，执行成功的回调函数
      if (this.state !== 'pending') return
      this.state = 'fulfilled'
      this.value = value
      this.onFulfilledCallBacks.forEach(fn=>fn(this.value))
    }
    reject(reason){
      if (this.state !== 'pending') return
      this.state = 'rejected'
      this.reason = reason
      this.onRejectedCallBacks.forEach(fn=>fn(this.reason))
    }
    then(onFulfilled,onRejected){
      if(typeof onFulfilled !=='function'){
        onFulfilled = function(value){
          return value
        }
      }
      if(typeof onRejected !== 'function'){
        onRejected = function(value){
          return value
        }
      }
      if(this.state === 'fulfilled'){
        setTimeout(()=>{
          onFulfilled(this.value)
        })
      }
      if(this.state === 'rejected'){
        setTimeout(()=>{
          onRejected(this.reason)
        })
      }
      if(this.state === 'pending'){
        this.onFulfilledCallBacks.push(value=>{
          setTimeout(()=>{
            onFulfilled(value)
          })
        })
        this.onRejectedCallBacks.push(reason=>{
          setTimeout(()=>{
            onRejected(reason)
          })
        })
      }
    }
  
}
module.exports = myPromise