javascript try/throw/catch syntaks?

JScript kode 

 try
 {
     if($("#signinusername").val() == "" && $("#signinpassword").val() == "")        throw "Please enter a username and password.";
     else if($("#signinusername").val() == "" && $("#signinpassword").val() != "")   throw "Please enter a username.";
     else if($("#signinusername").val() != "" && $("#signinpassword").val() == "")   throw "Please enter a password.";
     else
     {
         // check username and password with ajax
         var usr = $("#signinusername").val();
         var psw = $("#signinpassword").val();
         var ckie = $("#signincookie:checked").val();
         $.ajax(
         {
             url: signinpage,
             type: 'POST',
             data: {
                 action: "signin",
                 username: usr,
                 password: psw,
                 cookie: ckie
             },
             success: function(data)
             {
                 if(data == 1)
                 {
                     $("#signinstatus")
                     .removeClass("error")
                     .addClass("success")
                     .html("Signin OK. Redirecting...")
                     .slideDown('slow', 'easeInQuad');
                     setTimeout("location.href='"+redirect+"'", 3000);
                 }
                 else throw "Password and username does not match.";
             }
         });
     }
 }
 catch(err)
 {
     $("#signinstatus")
     .removeClass("success")
     .addClass("error")
     .html(err)
     .slideDown('slow', 'easeInQuad')
     .delay(3000)
     .slideUp('slow', 'easeOutQuad');
 }