Four steps for ajax to send asynchronous requests

This article mainly teaches you how to easily implement ajax to send asynchronous requests. It has certain reference value. Interested friends can refer to it. I hope it can help everyone better master the knowledge of ajax to send asynchronous requests.

ajax sends an asynchronous request for your reference. The specific content is as follows

The first step(get XMLHttpRequest)

ajax actually only needs to learn one object :XMLHttpRequest, if you master it, you will master ajax!!!

1. Get XMLHttpRequest

Most browsers support:var xmlHttp=new XMLHttpRequest();
IE6.0:var xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
IE5.0 to earlier versions of IE:var xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");

2. Write a function that creates an XMLHttpRequest object

function createXMLHttpRequest(){
     try{
     return new XMLHttpRequest();
     } catch(e){
     try{
     return new ActiveXObject(“Msxml2.XMLHTTP”);
    }catch(e){
     try{
      return new ActiveXObject(“Microsoft.XMLHTTP”);
     }catch(e){
       alert(“哥们儿,你用的是什么浏览器啊?”);
       throw e;
    }
    }
     } 
}

Second step(Open the connection with the server)

xmlHttp.open(): Used to open To connect to the server, it requires three parameters:

Request method: can be GET or POST
Requested URL: Specify server-side resources, for example:/day23_1/AServlet
Whether the request is asynchronous : If true, it means sending an asynchronous request, otherwise a synchronous request

##xmlHttp.open("GET","/day23_1/AServlet",true);//For example

The third step(Send request)

If it is a POST request, as follows

xmlHttp.send(“username=zhangSan&password=123”);

Four steps:

The xmlHttp object has a total of 5 states

1: The request has started, the open() method has been called, but the send() method has not been called yet
2: The request is sent in completion status, the send() method has been called
3: Start reading the server response
4: End reading the server response (usually we only care about the last status!!!)

var state = xmlHttp.readyState;//可能是0、1、2、3、4

var status=xmlHttp.status;//例如200、404、500

var content=xmlHttp.responseText;//得到服务器的响应的文本格式的内容(这更通用)
var content=xmlHttp.responseXML;//得到服务器的响应的xml响应的内容,它是document对象了!

xmlHttp.onreadystatechange = function(){

  //xmlHttp的5种状态都会调用本方法
    if(xmlHttp.readyState ==4 && xmlHttp.status == 200){

  //双重判断:判断是否为4状态,而且还要判断是否为200
      var text=xmlHttp.responseText;
     } 
};

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