js ajax 相关知识

创建 XMLHttpRequest 对象

不同的浏览器使用不同的方法来创建 XMLHttpRequest 对象。

Internet Explorer 使用 ActiveXObject。

其他浏览器使用名为 XMLHttpRequest 的 JavaScript 内建对象。

要克服这个问题，可以使用这段简单的代码：

下面的例子试图加载微软最新版本的 "Msxml2.XMLHTTP"，在 Internet Explorer 6 中可用，如果无法加载，则后退到 "Microsoft.XMLHTTP"，在 Internet Explorer 5.5 及其后版本中可用。

function GetXmlHttpObject()
{
var xmlHttp=null;


try
 {
 // Firefox, Opera 8.0+, Safari
 xmlHttp=new XMLHttpRequest();
 }
catch (e)
 {
 // Internet Explorer
 try
  {
  xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
  }
 catch (e)
  {
  xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
 }
return xmlHttp;
}

Ajax, sometimes written as AJAX (shorthand for asynchronous JavaScript and XML), is a group of interrelated web development techniques used on the client-side to create interactive web applications or rich Internet applications. With Ajax, web applications can retrieve data from the server asynchronously in the background without interfering with the display and behavior of the existing page. The use of Ajax has led to an increase in interactive animation on web pages and better quality of Web services thanks to the asynchronous mode. Data is retrieved using the XMLHttpRequest object. Despite the name, the use of JavaScript and XML is not actually required, nor do the requests need to be asynchronous.

JSON (JavaScript Object Notation) is a lightweight data-interchange format. It is easy for humans to read and write. It is easy for machines to parse and generate. It is based on a subset of the JavaScript Programming Language, Standard ECMA-262 3rd Edition - December 1999. JSON is a text format that is completely language independent but uses conventions that are familiar to programmers of the C-family of languages, including C, C++, C#, Java, JavaScript, Perl, Python, and many others. These properties make JSON an ideal data-interchange language.

the JSONP approach incorporates the use of an encoded callback function passed between the client and server to allow the client to load JSON-encoded data from third-party domains and to notify the caller function upon completion, although this imposes some security risks and additional requirements upon the server.