xmlhttp.responseXML ajax problem

hi

i am send a request using ajax and in respone i am sending back valid and invalid

and when i am writing the below code it not work

if(xmlhttp.responseXML=="valid")
{
alert("valid");
}
else
{
alert("invalid");
}

Code (markup):

its always calling else part even if i am getting valid in response.

and is there any way to check if my javascript code in correct i mean any kind of debugging