AJAX —— new XMLHttpRequest

    1. Example of an ajax object  

  var ajax = new XMLHttpRequest()

    2. The ajax object opens the interface ajax.open (request method, interface address, asynchronous or not) Asynchronous: multiple threads run simultaneously (true) Synchronous: single thread runs sequentially (false)    

ajax.open('get','http://localhost:3000/list','true') 

    3.ajax object sends request

    ajax.send()

    4. Monitor whether the request is successful

    ajax.onreadystatechange = function(){
        if(ajax.readyState==4){
            if(ajax.status==200){
                //相应ajax.respose 默认json字符串 转化为json
                //json字符串转换为json对象 var res = JSON.parse(ajax.response)
                //json对象转换为json字符串 var res = JSON.stringify(ajax.response)
                var res = JSON.parse(ajax.response)
                console.log(res)
                console.log(ajax.response)
            }
        }
    }