promise是構造函數(shù)

   setInterval(() => {
            const n = Math.random()
            new Promise((resolve,reject) => {
                if (n < 0.5) {
                    resolve(n)
                } else {
                    reject('cuowule')
                }
            }).then( (data) => {
                console.log(data)
            }).catch( mes => {
                console.log(mes)
            })
            
        }, 3000);

promise封裝

function request1 (url,cb) {
    return new Promise((resolve, reject) => {
        var xml = new XMLHttpRequest()
        xml.addEventListener('load', function () {
            resolve(this.responseText)
        })
        xml.open('get', url)
        xml.send()
    })
}

  request1('http://jsonplaceholder.typicode.com/posts').then(function (data) {
      console.log(data)
  })

• async await
  await就是等待,等待await后面的函數(shù)執(zhí)行完畢，異步函數(shù)內部再往下執(zhí)行

function request1 (url) {
    return new Promise((resolve, reject) => {
        var xml = new XMLHttpRequest()
        xml.addEventListener('load', function () {
            resolve(this.responseText)
        })
        xml.open('get', url)
        xml.send()
    })
}

  async function getData () {
      const a = await request1('http://jsonplaceholder.typicode.com/posts')
      console.log(a,111)
      const b = await request1('http://jsonplaceholder.typicode.com/posts')
      console.log(b, 222)
      const c = await request1('http://jsonplaceholder.typicode.com/posts')
      console.log(c, 333)
  }
  getData()
// 111 222 333