简述jQuery ajax的执行顺序

function TestAjax()
{
 var UserName = $("#txtUserName").val();
 $.ajax({
  url:"AjaxCheckUserName.htm",
  async:false,
  success:function(data){
   alert(data);
  }
 });
 alert('Test');
 $.ajax({
  url:"AjaxHandler.ashx",
  async:false,
  data:"UserName=" + UserName,
  success:function(data){
   $("#divAjax").html(data);
  },
  error:function(msg){
   alert(msg.responseText);
  }
 });
}