说明：  AJAX入门之XMLHttpRequest慨述 　　在使用XMLHttpRequest对象发送请求和处理响应之前，必须先用JavaScript创建一个XMLHttpRequest对象。由于XMLHttpRequest不是一个W3C标准，所以可以采用多种方法使用JavaScript来创建XMLHttpRequest的实例。 Internet Explorer把XMLHttpRequest实现为一个ActiveX对象，其他浏览器（如Firefox、Safari和Opera）把它实现为一个本地JavaScript对象。由于存在这些差别，JavaScript代码中必须包含有关的逻辑，从而使用ActiveX技术或者使用本地JavaScript对象技术来创建XMLHttpRequest的一个实例。
(AJAX entry of XMLHttpRequest outlined in the use of XMLHttpRequest object to send requests to respond to and deal with before using JavaScript to create a XMLHttpRequest object. As the XMLHttpRequest is not a W3C standard, it can be used many ways to use JavaScript to create XMLHttpRequest instances. Internet Explorer to XMLHttpRequest as an ActiveX object to achieve, other browsers (such as Firefox, Safari and Opera) put it to achieve for a local JavaScript object. Because of these differences, JavaScript code must contain the relevant logic, and thus the use of ActiveX technology or the use of local JavaScript object technology to create an instance of XMLHttpRequest.)