Ajax: Asynchronous communication technology in JavaScript

Ajax (Asynchronous JavaScript and XML) is an asynchronous communication technology used in JavaScript. It allows a web page to update part of the page content by exchanging data with the server without refreshing the entire page. This article will introduce in detail the principles, usage and corresponding source code examples of Ajax.

Ajax principles

The core principle of Ajax is to achieve asynchronous communication with the server through JavaScript's XMLHttpRequest object. When part of the web page needs to be updated, the JavaScript code creates an XMLHttpRequest object and uses this object to communicate with the server. After the server-side processing is completed, the response data is sent back to the client, and the JavaScript code updates the corresponding part of the page based on the response data.

How to use Ajax

Here are the basic steps for asynchronous communication using Ajax:

1. Create an XMLHttpRequest object: Use JavaScript code to create an XMLHttpRequest object.

2. Set a callback function: Set a callback function for the onreadystatechange event of the XMLHttpRequest object, which will be called when the communication state changes.

3. Send a request: Use the open() and send() methods of the XMLHttpRequest object to send the request to the server.

4. Processing the response: In the callback function, determine the communication status and results through the readyState and status attributes of the XMLHttpRequest object, and then process the server's response data as needed.

Here is an example of using Ajax to get server data and update the page:

// 创建XMLHttpRequest对象