fetch和ajax的功能相似，都是实现请求，但是多少有些兼容性问题。

    fetch优点：
    1.语法简洁，更加语义化
    2、基于标准 Promise 实现，支持 async/await

    下面是封装好的fetch(有处理兼容性)：

    1. export default async(url = '', data = {}, type = 'GET', method = 'fetch') => {
    2.     type = type.toUpperCase();
    3.     url = baseUrl + url;
    5.     if (type == 'GET') {
    6.         let dataStr = ''; //数据拼接字符串
    7.         Object.keys(data).forEach(key => {
    8.             dataStr += key + '=' + data[key] + '&';
    9.         })
    11.         if (dataStr !== '') {
    12.             dataStr = dataStr.substr(0, dataStr.lastIndexOf('&'));
    13.             url = url + '?' + dataStr;
    14.         }
    15.     }
    17.     if (window.fetch && method == 'fetch') {
    18.         let requestConfig = {
    19.             credentials: 'include',
    20.             method: type,
    21.             headers: {
    22.                 'Accept': 'application/json',
    23.                 'Content-Type': 'application/json'
    24.             },
    25.             mode: "cors",  //cors用于跨域请求
    26.             cache: "force-cache"
    27.         }
    29.         if (type == 'POST') {
    30.             Object.defineProperty(requestConfig, 'body', {
    31.                 value: JSON.stringify(data)
    32.             })
    33.         }
    35.         try {
    36.             const response = await fetch(url, requestConfig);
    37.             const responseJson = await response.json();
    38.             return responseJson
    39.         } catch (error) {
    40.             throw new Error(error)
    41.         }
    42.     } else {
    43.         return new Promise((resolve, reject) => {
    44.             let requestObj;
    45.             if (window.XMLHttpRequest) {
    46.                 requestObj = new XMLHttpRequest();
    47.             } else {
    48.                 requestObj = new ActiveXObject;
    49.             }
    51.             let sendData = '';
    52.             if (type == 'POST') {
    53.                 sendData = JSON.stringify(data);
    54.             }
    56.             requestObj.open(type, url, true);
    57.             requestObj.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    58.             requestObj.send(sendData);
    60.             requestObj.onreadystatechange = () => {
    61.                 if (requestObj.readyState == 4) {
    62.                     if (requestObj.status == 200) {
    63.                         let obj = requestObj.response
    64.                         if (typeof obj !== 'object') {
    65.                             obj = JSON.parse(obj);
    66.                         }
    67.                         resolve(obj)
    68.                     } else {
    69.                         reject(requestObj)
    70.                     }
    71.                 }
    72.             }
    73.         })
    74.     }
    75. }

    详情请看这里：这里,还有这里