完整Ajax的七个步骤
取得XMLHttpRequest对象
IE7+、Firefox、Opera、Chrome和Safari都支持原生的XHR对象，创建方法：
var xhr = new XMLHttpRequest();
IE6之前的使用ActiveXObject，需要传入正确的参数。
function createXHR(){
if (typeof XMLHttpRequest != "undefined"){
return new XMLHttpRequest();
} else if (typeof ActiveXObject != "undefined"){
if (typeof arguments.callee.activeXString != "string"){
var versions = [ "MSXML2.XMLHttp.6.0", "MSXML2.XMLHttp.3.0",
"MSXML2.XMLHttp"],
i, len;
for (i=0,len=versions.length; i < len; i++){
try {
new ActiveXObject(versions[i]);
arguments.callee.activeXString = versions[i];
break;
} catch (ex){
//跳过
}
}
}
return new ActiveXObject(arguments.callee.activeXString);
} else {
throw new Error("No XHR object available.");
}
}
原文：http://www.cnblogs.com/ElenaSun-FM/p/4897165.html