promise链式调用实现1

一、then方法中，onFulfilled方法和onRejected方法的调用会生成一个新的promise实例并返回，这两个方法返回的值会存为x
二、保存的x需要进行判断，用于改变新promise实例的状态。

const PENDING = 'pending';
const FULFILLED = 'fulfilled';
const REJECTED = 'rejected';
function resolvePromise(promise2,x,resolve,reject){
    console.log(promise2,x)
}
class MPromise{
    constructor(excuter){
      this.status = PENDING;
    this.value = undefined;
    this.reason = undefined;
    this.onFulfilledCallbacks = []
    this.onRejectedCallbacks = []
    //声明resole和reject函数，这里用箭头函数可以避免this指向改变，单独方法调用，this指向window或undefined
    const resolve = (val) =>{
        if(this.status === PENDING){
          this.value = val
        this.status = FULFILLED
        this.onFulfilledCallbacks.forEach((fn)=> fn())
      }
    }
    const reject = (reason) =>{
        if(this.status === PENDING){
          this.reason = reason
        this.status = REJECTED
        this.onRejectedCallbacks.forEach((fn)=> fn())
      }
    }
    try{
        executor(resolve, reject)
    }catch(error){
        reject(error)
    }
  }
  //then方法核心部分是，判断promise当前status，做出不同操作
  //function class里面添加原型方法，不用写function关键字
  then(onFulfilled, onRejected){
        let promise2 = new Promise((resolve,reject)=>{
      //如果是pending状态，收集相关方法，等状态改变时再调用
        if(this.status === PENDING){
          //之前搜集回调时，没有传value和reason
          this.onFulfilledCallbacks.push(()=>{
            try{
              let x = onFulfilled(this.value)
            }catch(error){
              reject(error)
            }
          })
          this.onRejectedCallbacks.push(()=>{
            try{
              let x = onRejected(this.reason)
            }catch(error){
              reject(error)
            }
          })
        }
        if(this.status === FULFIILED){
          setTimeout(()=>{
            try{
              let x = onFulfilled(this.value)
              //这里要将新promise实例传到函数中，但是此处是在new Promise的executor中
              //需要将onFulfilled和onRejected写成异步方法，可以写成宏任务或者微任务，
              //源码中写成了微任务
              resolvePromise(promise2,x,resolve,reject)
            }catch(error){
              reject(error)
            }
          },0)
        }
        if(this.status === REJECTED){
          setTimeout(()=>{
            try{
                let x = onRjected(this.reason)
              resolvePromise(promise2,x,resolve,reject)
            }catch(error){
              reject(error)
            }
          },0)
        }
    })
    return promise2
  }
}