Problem With 'undefined' Object In Server Side JavaScript

I'm displaying a loggin page.

If the user entered an invalid UserName/Password I display the page again.
When this happens I want to give the user a different message then the one displayed when the page was initially displayed.

I use the Response.Redirect method from the page checking the submition.
When I do that I call the page with a parameter like this Response.Redirect("Page.asp?FROM=1").

When I get the parameter in the page I know I need to show a different message.

The problem is that if the page is called without the parameter, I
cannot determine that the parameter is missing.

The codse is very simple:

from_flag = Request.QueryString("FROM")

Now is the problem -

if I use if(from_flag == 'undefined') - this condition return
false.
but if I display from_flag using Response.Write(from_flag) I get "undefined"

if I use if(typeof from_flag == "undefined") - this does not work since the typeof return "object" in both cases when I call the page with the parameter or without the parameter.

also I tried to comnpare with null does not work.

By the way if I use VBScript as the script lang - it works fine I just need to compare the from_flag with vbNullString and all is well....