How do I test for the presence of an active open document in Photoshop?

Question

I’m new to Adobe scripting, and one of my first scripts deals with the activeDocument in Photoshop.
If there is no activeDocument, I want to display a message and terminate the script.  My efforts
so far have been met with frustration.  While developing the script in ExtendScript Toolkit,
I used the  test expression "activeDocument in app", and everything worked as expected. And when I installed the script
in the Photoshop scripts folder and ran it from Photoshop with a document open, it also worked.
But when I ran the script from Photoshop with no document open, it failed.
Moreover, after that,the script failed when I tried to run it in ESTK.
I did some testing and found that my test expression, "activeDocument in app"
was returning true, even though no document was open.
Still in ESTK, I replaced the “app” with ”Application” in the test expression.  That worked when
I stepped through the script.  But when I tried to run it without stopping, it stopped at the
test expression, and displayed the error message, “Application is undefined.”
So how do I test for the presence of an open document in Photoshop ??

Answer 1

After posting this, I discovered the Photoshop Scripting Forum, and I found my answer there.—
I should have used “app.Documents.length” as my test expression.