Implement ajax to send asynchronous requests (graphic tutorial)

This article mainly teaches you how to easily implement ajax to send asynchronous requests, which has certain reference value. Interested friends can refer to

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

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 uses 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;
    }
    }
     } 
}

The second step(Open the connection with the server)

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

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

Parameters: Yes Request body content! If it is a GET request, null must be given.
If it is a POST request, as follows

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 be written 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;
     } 
};

The above is what I compiled for everyone. I hope it will be helpful to everyone in the future.

Related articles:

Ajax implementation of saving and reading uploaded images to the background (graphic tutorial)

Struts2 and Ajax data interaction (graphic tutorial)

The solution to the problem that the Ajax request is sent successfully but does not enter the success (graphic tutorial)

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