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)

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, 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;
     } 
};

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