使用promise写一个简单的ajax案例: 

    1.   function ajax (url) {
    2.     return new Promise(function (resolve, reject) {
    3.       let xhr = new XMLHttpRequest()
    4.       xhr.open('GET', url)
    5.       xhr.responseType = 'json'
    6.       xhr.onload = function () {
    7.         if (this.status + '' === '200') {
    8.           resolve(this.response)
    9.           console.log(this.response)
    10.         } else {
    11.           reject(new Error(this.responseText))
    12.           console.log(this.responseText)
    13.         }
    14.       }
    15.       xhr.send()
    16.     })
    17.   }
    18.   ajax('./ajaxTest.json')