AJAX原理以及axios、fetch区别实例详解

var xhr = window.XMLHttpRequest?new XMLHttpRequest():new ActiveXObject('Microsoft.XMLHTTP');// 兼容IE6及以下版本

xhr.open('get', url, true)

xhr.send(null); // 严谨写法

xhr.onreadystatechange = function(){
    if(xhr.readyState == 4){
        if(xhr.status == 200){
            success(xhr.responseText);
        } else { 
            
            fail && fail(xhr.status);
        }
    }
}

// promise 封装实现：
function getJSON(url) {
  // 创建一个 promise 对象
  let promise = new Promise(function(resolve, reject) {
    let xhr = new XMLHttpRequest();

    // 新建一个 http 请求
    xhr.open("GET", url, true);

    // 设置状态的监听函数
    xhr.onreadystatechange = function() {
      if (this.readyState !== 4) return;

      // 当请求成功或失败时，改变 promise 的状态
      if (this.status === 200) {
        resolve(this.response);
      } else {
        reject(new Error(this.statusText));
      }
    };

    // 设置错误监听函数
    xhr.onerror = function() {
      reject(new Error(this.statusText));
    };

    // 设置响应的数据类型
    xhr.responseType = "json";

    // 设置请求头信息
    xhr.setRequestHeader("Accept", "application/json");

    // 发送 http 请求
    xhr.send(null);
  });
  return promise;
}

// 用户登录 -> 登录成功 -> 获取用户信息

$.ajax({
    url: 'http://127.0.0.1:8888/user/login',
    method: 'post',
    data: Qs.stringify({
        account: '18310612838',
        passWord: md5('1234567890')
    }),
    success(result) {
        if (result.code === 0) {
            // 登录成功
            $.ajax({
                url: 'http://127.0.0.1:8888/user/list',
                method: 'get',
                success(result) {
                    console.log(result);
                }
            });
        }
    }
});

axios({
    method: 'post',
    url: '/user/login',
    data: {
        username: 'name',
        password: 'password'
    }
})
.then(function (response) {
    console.log(response);
})
.catch(function (error) {
    console.log(error);
});
// 或使用 async await
(async function () {
    let result1 = await axios.post('/user/login', {
        username: 'name',
        password: 'password'
    });
    let result2 = await axios.get('/user/list');
    console.log(result1, result2);
})(); 

try {
  let response = await fetch(url, options);
  let data = response.json();
  console.log(data);
} catch(e) {
  console.log("Oops, error", e);
}

(async function () {
    let result = await fetch('http://127.0.0.1:8888/user/login', {
        method: 'post',
        headers: {
            'Content-Type': 'application/x-www-fORM-urlencoded'
        },
        body: Qs.stringify({
            name: 'name',
            password: 'password'
        })
    })
    let data = result.json();
    console.log(data)
​
    let result2 = await fetch('http://127.0.0.1:8888/user/list').then(response => {
        return response.json();
    });
    console.log(result2);
})();