Ajax同步和异步调用

ajax函数(同步方式)：

function queryIsSameLevel(){
		var ID_MT_OPERATIONTYPE = document.getElementById('ID_MT_OPERATIONTYPE').value;
		var url = '<%=request.getScheme() + "://" + request.getServerName() + ":" + request.getServerPort() + request.getContextPath() %>'

		var executeURL = url+"/common/mt/isSameLevel.do?id="+ID_MT_OPERATIONTYPE;
		
		var xmlhttp;
		
		if (window.XMLHttpRequest){
		  xmlhttp= new XMLHttpRequest();
		}else{
		  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
	  	}

	  	//同步方式
		xmlhttp.open("POST",executeURL,false);
		xmlhttp.send();  
		if(xmlhttp.readyState==4 && xmlhttp.status==200){
			//alert("你大爷" + xmlhttp.responseText);
			responseTextVal = xmlhttp.responseText;
			responseTextVal = responseTextVal.replace('{','');
			responseTextVal = responseTextVal.replace('}','');
			var  index = responseTextVal.indexOf(":");
			responseTextVal = responseTextVal.substring(index+1);
			responseTextVal = responseTextVal.replace('"','');
			responseTextVal = responseTextVal.replace('"','');
		}
	}


异步方式：

function queryIsSameLevel(){
		var ID_MT_OPERATIONTYPE = document.getElementById('ID_MT_OPERATIONTYPE').value;
		var url = '<%=request.getScheme() + "://" + request.getServerName() + ":" + request.getServerPort() + request.getContextPath() %>'
		var executeURL = url+"/common/mt/isSameLevel.do?id="+ID_MT_OPERATIONTYPE;
		var xmlhttp;
		if (window.XMLHttpRequest){
		  xmlhttp= new XMLHttpRequest();
		}else{
		  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
	  	}

	  	//异步方式
		xmlhttp.open("POST",executeURL,true);
		xmlhttp.send();  
		//异步方式添加回调函数
		xmlhttp.onreadystatechange=function(){
			if(xmlhttp.readyState==4 && xmlhttp.status==200){
				//alert("你大爷" + xmlhttp.responseText);
				responseTextVal = xmlhttp.responseText;
				responseTextVal = responseTextVal.replace('{','');
				responseTextVal = responseTextVal.replace('}','');
				var  index = responseTextVal.indexOf(":");
				responseTextVal = responseTextVal.substring(index+1);
				responseTextVal = responseTextVal.replace('"','');
				responseTextVal = responseTextVal.replace('"','');
			}
		}
	}


如果有一个方法B()调用上述两个方法queryIsSameLevel()，

异步方式：有可能回调函数的执行比B函数中的代码执行要晚

同步的方式：等方法queryIsSameLevel()全部执行完后再去执行方法B()中的代码
            同步方式没有回调函数，执行完send()方法后就进行状态的判断


参考资料：
http://hi.baidu.com/iamgongjunhao/blog/item/e4357a501ae429748535242f.html