三种方式获取XMLHttpRequest对象

//1 
function getXMLHttpRequest() { 
var xmlHttpReq; 
try { // Firefox, Opera 8.0+, Safari 
xmlHttpReq = new XMLHttpRequest(); 
} catch (e) { 
try {// Internet Explorer 
xmlHttpReq = new ActiveXObject("Msxml2.XMLHTTP"); 
} catch (e) { 
try { 
xmlHttpReq = new ActiveXObject("Microsoft.XMLHTTP"); 
} catch (e) { 
} 
} 
} 
return xmlHttpReq; 
} 

//2 
function getXMLHttpRequest() { 
var xmlHttpReq = null; 
if (window.ActiveXObject) {// Internet Explorer 
xmlHttpReq = new ActiveXObject("MSXML2.XMLHTTP.3.0"); 
} else if (window.XMLHttpRequest) { 
xmlHttpReq = new XMLHttpRequest(); 
} 
return xmlHttpReq; 
} 

//3 
function getXMLHttpRequest() { 
var xmlHttpReq = null; 
if (window.XMLHttpRequest) {// Mozilla Firefox, Opera 8.0+, Safari 
xmlHttpReq = new XMLHttpRequest(); 
} else { 
if (window.ActiveXObject) {// Internet Explorer 
try { 
xmlHttpReq = new ActiveXObject("Microsoft.XMLHTTP"); 
} catch (e) { 
try {// Internet Explorer 
xmlHttpReq = new ActiveXObject("Msxml2.XMLHTTP"); 
} catch (e) { 
} 
} 
} 
} 
return xmlHttpReq; 
}