3.Ajax迭代

1.原生ajax

var url = "http://192.168.4.18:8000/"
var xhr = new XMLHttpRequest();
xhr.open("get", url, true);
xhr.send();
xhr.onreadystatechange = function () {
  if (xhr.readyState == 4 && xhr.status == 200) {
    var result  = JSON.parse(xhr.responseText);
    console.log(result)
  }
}

2.回调函数封装ajax

//需要记住传参的顺序
function $ajax(url,success){
    var xhr = new XMLHttpRequest();
    xhr.open("get",url,true);
    xhr.send();
    xhr.onreadystatechange =  function(){
        if(xhr.readyState == 4  && xhr.status==200){
            var res = JSON.parse(xhr.responseText);
            success(res)
        }
    }
}

//封装成对象的形式不需要记住传参的顺序
function $ajax({url,success}){
    var xhr = new XMLHttpRequest();
    xhr.open("get",url,true);
    xhr.send();
    xhr.onreadystatechange =  function(){
        if(xhr.readyState == 4  && xhr.status==200){
            var res = JSON.parse(xhr.responseText);
            success(res)
        }
    }
}

3.jquery-ajax

        var url  = `http://47.108.197.28:8000/book`;
        $.ajax({
            url,
            success:res=>{
                console.log(res)
            }
        })

4.promise封装ajax

function http(url){
  return new Promise((resolve,reject)=>{
    $.ajax({
      url,
      type:"get",
      success:res=>{
        resolve(res)
      },
      error:err=>{
        reject(err);
      }
    })
  })
}