function () {
	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"];
			var i,len;
			for(i=0;len=versions.length;i < len;i++) {
				try {
					new ActiveXObject(versions[i]);
					arguments.callee.activeXString = versions[i];
					break;	
				} catch(ex) {
				
				}
			} 
		}
		return new ActiveXObject(arguments.callee.activeXString);
	} else {
		throw new Error("No XHR object available");
	}
}

var xhr = createXHR();
xhr.onreadystateChange = function(){
	if(xhr.readyState == 4) {
		if((xhr.status >= 200 && xhr.status < 300) || xhr.status == 304) {
			console.log(xhr.responseText);
		} else {
			console.log("Resuest was unsuccessful: " + xhr.status);
		}
	}
}
xhr.open('get','example.php',false);
xhr.setRequestHeader('MyHeader','MyValue');
xhr.send(null);

var myHeader = xhr.getResponseHeader("MyHeader");//获得相应响应头部信息
var allHeaders = xhr.getAllResponseHeaders();//获得全部头部信息的长字符串