1 在文件夹里面新建一个js
   <script>
        var url = 'http://47.108.197.28:8000/book'
        //var url ="http:47.108.197.28:3000/banner"
        function $ajax(url,success){
            var xhr = new XMLHttpRequest();
            xhr.open("get",url,true);
            xhr.send();
            xhr.onreadystatechange = function(){
                if(xhr.responseText ==4 && xhr.status==200){
                    var res = JSON.parse(xhr.responseText);
                    success(res)
                }
            }
        }
        $ajax(url,res=>{
            console.log(res)
        })

回调函数

<script>
        // function $ajax(callback){
        //     var res = "hello word"
        //     callback(res);
        // }
        // var s =$ajax(res=>{
        //     console.log(res)
        // })
        //写在本子上
        function $ajax({
            success
        }){
        var res = "hello word"
        success(res);
        }
        $ajax({
            success:res=>{
                console.log(res)
            }
        })
    </script>

示例

 <script>
        /*
        函数的参数
        */
        // function ajax(url,method,c){
        //     console.log(url);
        //     console.log(method);
        //     console.log(c)
        // }
        // ajax("http","get",1)
        // ajax(1,"http","get")
            //如果参数过多，可以传对象
        function ajax({
            url,
            method,
            c
        }){
            console.log(url);
            console.log(method);
            console.log(c)
        }
        ajax({
            method:"get"
        })
    </script>

<script>
        /*
        如何实现ajax
        */
        var url = 'http://47.108.197.28:8000/book'
        // var success = function(res){
        //     console.log(res)
        // }
        function go(){
            var res = "hello word"
            success(res)
        }
        go(res=>{
            console.log(res)
        })
    </script>

练习

 <script>
        var banner = 'http://47.108.197.28:3000/banner'
        var xhr  =new XMLHttpRequest();
        xhr.open("get",banner,true)
        xhr.send();
        xhr.onreadystatechange = function(){
            if(xhr.readyState==4 && xhr.status==200){
            console.log(xhr.responseText)
            var res = JSON.parse(xhr.responseText)
            console.log(res)
        }
        }
    </script>