It only takes four steps to implement ajax to send asynchronous requests

This time I will bring you just four steps to implement ajax to send asynchronous requests, and four steps to implement ajax to send asynchronous requests. What are the precautions?The following is a practical case, let's take a look.

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 and earlier versions of IE:var xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");

2. Write the function that creates the 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;
    }
    }
     } 
}

The second step(Open the connection with the server)

xmlHttp.open(): used to open the connection with the server, it Three parameters are required:

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

Step Three(Send Request)

xmlHttp.send(null): If not given, some browsers may not be able to send!

Parameter: It is the request body Content! If it is a GET request, null must be given.
If it is a POST request, the following is

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

Four steps:

Register a listener on an event of the xmlHttp object: onreadystatechange
The xmlHttp object has a total of 5 states

0: Initialization is not completed, just created XMLHttpRequest object, the open() method has not been called yet
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!!!)

Get the status of the xmlHttp object

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

Get the status code of the server response (200: Success 304: Status has not changed 404 500: Server error)

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

Get the content of the server response

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!

So the listener should Write like this

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;
     } 
};

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the php Chinese website!

Recommended reading:

Ajax+Servlet to implement non-refresh drop-down linkage (with code)

ajax to obtain json data How to use

for undefined

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