function ajax (url) {
  return new Promise((resolve, reject) => {
    let xhr = new XMLHttpRequest()
    xhr.onreadystatechange = function () {
      if (this.readyState === 4) {
        if (this.status === 200) {
          console.log('resolve')
          resolve(this.responseText)
        } 
 
else {
          console.log('error',)
          reject(new Error('reject error'))
        }
      }
    }
    xhr.open('get', url)
    xhr.send()
  })

}

  <script>
    function send () {
      ajax('http://localhost:555/vanity')
        .then(data => { console.log(data); return ajax('http://localhost:555/vanity
 
') }, err => console.log(err))
        .then(data => { console.log(data); return ajax('http://localhost:555/vanity') }, err => console.log(err))
        .then(data => { console.log(data); return ajax('http://localhost:555/vanity') }, err => console.log(err))
        .then(data => console.log(data), err => console.log(err))
    }
  
 
</script>

class MyPromise {
  status = 'pending'  // 初始狀態
  value = undefined
  reason = undefined
  onfulfilled = undefined
  onrejected = undefined
  constructor(executor) {
    console.log('constructor')
    executor(this.resolve, this.reject)
  }

  resolve = value => {
    console.log('resolve')
    if(this.status !== 'pending') return
    this.status = 'fulfilled'
    this.value = value
    this.onfulfilled && this.onfulfilled(this.value)
  }

  reject = reason => {
    console.log('reject')
    if(this.status !== 'pending') return
    this.status = 'rejected'
    this.reason = reason
    this.onrejected && this.onrejected(this.reason)
  }

  then(onfulfilled, onrejected) {
    console.log('call then')
    if(this.status === 'fulfilled') {
      console.log('then fulfilled')
      onfulfilled(this.value)
    } else if(this.status === 'rejected') {
      console.log('then reject')
      onrejected(this.reason)
    } else {  // 非同步回撥必定到此處
      console.log('then pending')
      this.onfulfilled = onfulfilled  // 為當前Promise物件繫結then的成功回撥
      this.onrejected = onrejected  // 為當前Promise物件繫結then的失敗回撥
    }
  }
}

fs.readFile

import fs from 'fs'

let pp = new MyPromise((resolve, reject) => {
  fs.readFile('./a.txt', 'utf8', (err, data) => {
    if(err) {  // readFile非同步回撥進入EventLoop, 必定後於then執行
      reject(err)
    } else {
      resolve(data)
    }
  })
})
// 繫結成功和失敗的執行函式
pp.then(data => console.log(data), err => console.log(err))

new Date().getTime() 模擬

let promise = new MyPromise((resolve, reject) => {
  setTimeout(() => {
    const timestamp = new Date().getTime()
    if(timestamp % 2) {
      resolve(`odd ${timestamp}`)
    } else {
      reject(`even ${timestamp}`)
    }
  },1000)
})

promise.then(value => {
  console.log('success', value)
}, err => {
  console.log('fail', err)
})