javascript中settimeout和异步引发思考

var t = true;
setTimeout(function(){ t = false; }, 1000);
while(t){}
alert('end');

 setTimeout(function(){while(true){}},1000);
setTimeout(function(){alert('end 2');},2000);
setTimeout(function(){alert('end 1');},100);
alert('end');

 var xmlReq = createXMLHTTP();//创建一个xmlhttprequest对象
function testAsynRequest(){
 var url = "/AsyncHandler.ashx?action=ajax";
 xmlReq.open("post", url, true);
 xmlReq.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
 xmlReq.onreadystatechange = function(){
  if(xmlReq.readyState == 4){
   if(xmlReq.status == 200){
    var jsonData = eval('(' + xmlReq.responseText + ')');
    alert(jsonData.message);
   }else if(xmlReq.status == 404){
    alert("Requested URL is not found.");
   }else if(xmlReq.status == 403){
    alert("Access denied.");
   }else{
    alert("status is " + xmlReq.status);
   }
  }
 };
 xmlReq.send(null);
}
testAsynRequest();//1秒后调用回调函数
while(true){}

 private void ProcessAjaxRequest(HttpContext context){
 string action = context.Request["ajax"];
 Thread.Sleep(1000);//等1秒
 string jsonObject = "{"message":"" + action + ""}";
 context.Response.Write(jsonObject);
}