javascript一个无懈可击的实例化XMLHttpRequest的方法

function getHTTPRequest() 
{ 
var xhr = false; 
if (window.XMLHttpRequest) 
xhr = new XMLHttpRequest(); //IE除外的浏览器 
else if (window.ActiveXObject) 
{ 
try 
{ 
xhr = new ActiveXObject("Msxm12.XMLHTTP");//最新版的ActiveX对象 
} 
catch(e) 
{ 
try 
{ 
xhr = new ActiveXObject("Microsoft.XMLHTTP"); 
} 
catch(e) 
{ 
xhr = false; 
} 
} 
} 
}

接下来是一个应用的实例：

//完整的GET请求 
var request = getHTTPRequest(); 
if(request) 
{ 
request.onreadystatechange = dosomething; 
request.open("GET","file.doc",true); 
request.send(null); 
}