How would I make a screenmate

You know this little app that a cat would pop up and follow you cursor, how would I make something like that?

GDevelop games cannot have transparent scene backgrounds that show the desktop beneath them. So I don’t believe this can be accomplished with the engine.

You could always make a virtual pet style game, but the window will be visible.

Is there a way with javascript?

There is not a way to make the background transparent, no.