手写一个Promise ajax

server

const express = require('express')
const app = express()
const bodyParser = require('body-parser')
// app.use(bodyParser.urlencoded({ extended: false }))
app.use(bodyParser.json())
// 解决跨域的问题
app.all('*', function(req, res, next) {
  res.header("Access-Control-Allow-Origin", "*");
  // res.header("Access-Control-Allow-Headers", "Content-Type");
  // res.header("Access-Control-Allow-Methods","PUT,POST,GET,DELETE,OPTIONS");
  // res.header("X-Powered-By",' 3.2.1')
  // res.header("Content-Type", "application/json;charset=utf-8");
  next();
});
app.get('/user', function (req, res) {
  res.send('Get请求成功')
})
app.post('/user', (req, res) => {
  console.log(req.body);
  res.send('Post请求成功')
})
app.listen(3000, () => {
  console.log('服务器启动成功');
})

client

 function ajax(type, params) {
    let { url, data } = params
    return new Promise((reslove, reject) => {
      let xhr = new XMLHttpRequest()
      if (type === 'get') {
        data = null
        xhr.open('get', url)
      } else if (type === 'post') {
        xhr.open('post', url, true)
        // xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded')
        xhr.setRequestHeader('Content-Type', 'application/json')
      }
      xhr.onreadystatechange = function () {
        if (this.readyState == 4) {
          if (this.status === 200) {
            reslove(this.responseText)
          } else {
            reject('请求信息有误')
          }
        }
      }
      xhr.send(JSON.stringify(data))
    })
  }
  ajax('get', {
    url: 'http://localhost:3000/user'
  }).then(res => {
    console.log(res);
  }).catch(res => {
    console.log(res);
  })
  // ajax('post', {
  //   url: 'http://localhost:3000/user',
  //   data: {
  //     username: 'zs',
  //     age: 20
  //   }
  // }).then(res => {
  //   // console.log(res);
  // }).catch(res => {
  //   console.log(res);
  // })