How to make an HTTP request 异步 JavaScript 和 XML

小结：

1、解耦数据交互层、展示层

By decoupling the data interchange layer from the presentation layer, Ajax allows Web pages, and by extension Web applications, to change content dynamically without the need to reload the entire page.^[3]

https://developer.mozilla.org/en-US/docs/AJAX/Getting_Started

In order to make an HTTP request to the server using JavaScript, you need an instance of a class that provides this functionality. This is where XMLHttpRequest comes in. Such a class was originally introduced in Internet Explorer as an ActiveX object called XMLHTTP. Then, Mozilla, Safari and other browsers followed, implementing an XMLHttpRequest class that supports the methods and properties of Microsoft's original ActiveX object. Meanwhile Microsoft has implemented XMLHttpRequest as well.

为了用JavaScript向服务器发送一个HTTP请求, 需要一个具备这种功能的类实例. 这样的类首先由Internet Explorer以ActiveX对象引入, 被称为XMLHTTP. 后来Mozilla, Safari 和其他浏览器纷纷仿效, 提供了XMLHttpRequest类,它支持微软的ActiveX对象所提供的方法和属性.

<script type="text/javascript">
<!--
    if(window.XMLHttpRequest){
        httpRequest = new XMLHttpRequest();
    } else if (window.ActiveObject)
    {
        httpRequest = new ActiveObject("Microsoft.XMLHTTP");
    }

//-->
</script>

ajax（AJAX开发） - 搜狗百科 https://baike.sogou.com/v25064.htm?fromTitle=Ajax

Ajax 即“AsynchronousJavascriptAndXML”（异步 JavaScript 和 XML），是指一种创建交互式网页应用的网页开发技术。

Ajax = 异步JavaScript和XML（标准通用标记语言的子集）。

Ajax 是一种用于创建快速动态网页的技术。

Ajax 是一种在无需重新加载整个网页的情况下，能够更新部分网页的技术。^[1]

通过在后台与服务器进行少量数据交换，Ajax 可以使网页实现异步更新。这意味着可以在不重新加载整个网页的情况下，对网页的某部分进行更新。

传统的网页（不使用 Ajax）如果需要更新内容，必须重载整个网页页面

Ajax (programming) - Wikipedia https://en.wikipedia.org/wiki/Ajax_%28programming%29

Ajax (also AJAX /ˈeɪdʒæks/; short for "Asynchronous JavaScript And XML")^[1][2] is a set of Web development techniques using many Web technologies on the client side to createasynchronous Web applications. With Ajax, Web applications can send and retrieve data from a server asynchronously (in the background) without interfering with the display and behavior of the existing page. By decoupling the data interchange layer from the presentation layer, Ajax allows Web pages, and by extension Web applications, to change content dynamically without the need to reload the entire page.^[3] In practice, modern implementations commonly utilize JSON instead of XML due to the advantages of JSON being native to JavaScript.^[4]

Ajax is not a single technology, but rather a group of technologies. HTML and CSS can be used in combination to mark up and style information. The webpage can then be modified by JavaScript to dynamically display – and allow the user to interact with — the new information. The built-in XMLHttpRequest object within JavaScript is commonly used to execute Ajax on webpages allowing websites to load content onto the screen without refreshing the page. Ajax is not a new technology, or different language, just existing technologies used in new ways.

The term Ajax has come to represent a broad group of Web technologies that can be used to implement a Web application that communicates with a server in the background, without interfering with the current state of the page. In the article that coined the term Ajax,^[1][3] Jesse James Garrett explained that the following technologies are incorporated:

• HTML (or XHTML) and CSS for presentation
• The Document Object Model (DOM) for dynamic display of and interaction with data
• JSON or XML for the interchange of data, and XSLT for its manipulation
• The XMLHttpRequest object for asynchronous communication
• JavaScript to bring these technologies together

Since then, however, there have been a number of developments in the technologies used in an Ajax application, and in the definition of the term Ajax itself. XML is no longer required for data interchange and, therefore, XSLT is no longer required for the manipulation of data. JavaScript Object Notation (JSON) is often used as an alternative format for data interchange,^[15] although other formats such as preformatted HTML or plain text can also be used.^[16] A variety of popular JavaScript libraries, including JQuery, include abstractions to assist in executing Ajax requests.

Asynchronous HTML and HTTP (AHAH) involves using XMLHTTPRequest to retrieve (X)HTML fragments, which are then inserted directly into the Web page.