ES6-手写Promise

首先Promise有三个状态 PENDING、FULFILLED、REJECTED ，只能由PENDING改变为FULFILLED、REJECTED，并且只能改变一次
Promise接收一个函数，这个函数有两个参数resolve方法和reject方法
resolve将PENDING改变为FULFILLED
reject将PENDING改变为FULFILLED

这里还实现了then,all,rece方法

class MyPromise{
      static PENDING = '待定';
      static FULFILLED = '成功';
      static REJECTED  = '失败';
      constructor(func){
          this.status =  MyPromise.PENDING;
          this.result = null;
          this.resolveCallback = [];
          this.rejectCallback = [];
          try {
              func(this.resolve.bind(this),this.reject.bind(this));
          } catch (error) {
              this.reject(error)
          }
          
      }
      resolve(result){
          setTimeout(() => {
              if(this.status == MyPromise.PENDING){
                  this.status = MyPromise.FULFILLED;
                  this.result   =  result;
                  this.resolveCallback.forEach(callback =>{
                      callback(result)
                  })
              }
          });
      }
      reject(result){
          setTimeout(() => {
              if(this.status == MyPromise.PENDING){
                  this.status = MyPromise.REJECTED;
                  this.result   =  result;
                  this.rejectCallback.forEach(callback =>{
                      callback(result)
                  })
              }
          });
      }
      then(onFULFILLED,onREJECTED){
          return new MyPromise( (resolve,reject)=>{
              onFULFILLED = typeof onFULFILLED  === 'function'?onFULFILLED:()=>{}
              onREJECTED = typeof onREJECTED  === 'function'?onREJECTED:()=>{}
              if(this.status == MyPromise.PENDING){
                  this.resolveCallback.push(onFULFILLED)
                  this.rejectCallback.push(onREJECTED)
              }
              if(this.status ==  MyPromise.FULFILLED){
              setTimeout(()=>{
                  onFULFILLED(this.result)
              })
              }
              if(this.status == MyPromise.REJECTED){
              setTimeout(()=>{
                  onREJECTED(this.result)
              })
              }
          })
      }
      all(arr){
          if(!Array.isArray(arr)){
              throw new Error('不是数组')
          }
          return new MyPromise( (resolve,reject) =>{
              let resolveValues = [];
              let num = 0;
              arr.forEach((item,index)=>{
                  item.then( result =>{
                      resolveValues[index] = result;
                      num ++;
                      if(resolveValues.length == arr.length){
                        return  resolve(resolveValues)
                      }
                  },(error)=>{
                      return  reject(error)
                  })
              })
          })
      }
      rece(arr){
          if(!Array.isArray(arr)){
              throw new Error('不是数组')
          }
          return new MyPromise( (resolve,reject) =>{
              arr.forEach((item,index)=>{
                  item.then( result =>{
                      return  resolve(result)
                  },(error)=>{
                      return  reject(error)
                  })
              })
          })
      }
  }
  console.log('第一步')
  let MyPromise =  new MyPromise((resolve,reject)=>{
      console.log('第二步')
      resolve('第一次');
      console.log('第四步')
  });

  MyPromise.all([com1(),com2()]).then(res =>{
      console.log(res)
  },(err)=>{
      console.log(err)
  })

  function com1(){
      return new MyPromise( (resolve,reject)=>{
          // throw new Error('宝错了')
          setTimeout( ()=>{
              resolve('com1')
          })
      })
  }

  function com2(){
      return new MyPromise( (resolve,reject)=>{
          setTimeout( ()=>{
              resolve('com2')
          })
      })
  }
  
  // MyPromise.then(
  //     result => console.log(result),
  //     result => console.log(result.messages)
  // ).then(
  //     result => console.log(result),
  //     result => console.log(result.messages)
  // )
        ```