手写一个promise

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  <title>Document</title>
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<body>
  
</body>
<script>
  const PEDDING = 'pedding' 
  const RESOLVE = 'resolve' 
  const REJECT = 'reject'
  function MyPromise(fn){
    const that = this
    that.state = PEDDING
    that.value = null
    that.resolveCallbacks = []
    that.rejectCallbacks = []
    function resolve(value) {
      if(that.state === PEDDING) {
        that.state = RESOLVE
        that.value = value
        that.resolveCallbacks.map(cb => cb(that.value))
      }
    }
    function reject(value) {
      if(that.state === PEDDING) {
        that.state = REJECT
        that.value = value
        that.rejectCallbacks.map(cb => cb(that.value))
      } 
    }
    try {
      fn(resolve, reject)
    }catch(e){
      reject(e)
    } 
  }
  MyPromise.prototype.then = function(onFullfilled, onRejected){
    const that = this
    onFullfilled = typeof onFullfilled === 'function'? onFullfilled : v => v
    onRejected = typeof onRejected === 'function' ? onRejected : r => {
      throw r
    }  
    if (that.state === PEDDING) {
      that.resolveCallbacks.push(onFullfilled)
      that.rejectCallbacks.push(onRejected)
    }
    if(that.state === RESOLVE) {
      onFullfilled(that.value)
    }
    if(that.state === REJECT) {
      onRejected(that.value)
    }
  }
  console.log('script start')
  async function async1() {
    await async2()
    console.log('async1 end')
  }
  function async2() {
    console.log('async2 end')
  }
  async1()
  setTimeout(()=>{
    console.log('settimeout')
  }, 0)
  new MyPromise((resolve) => {
    console.log('promise start')
    resolve()
  }).then(()=>{
    console.log('promise1')
  })
  console.log('script end')

// script start => async2 end => promise start => promise1 => script end => async1 end => settimeout
</script>
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