What differences will happen if you use a rectangular baker to bake cookies instead of a tray? Hey so I have these rectangular bakers that I use to make lasagna. Unfortunately I don't bake cookies that often so I don't have a standard cookie tray. I decided to bake some cookies though so I opted for using those since those are what I have in my disposal. See below



I just popped them into the oven but I actually didn't consider if this would affect how long they need to cook for since they have that lip that surround the cookies. Will they turn out okay? If I don't get an answer in the next 8 or so minutes I can post what will happen for science.
They should be fine.

It's possible that they'll be a bit gooey, or just a touch underdone.  The high sides of the pan may shield the cookies from the heat, just a little bit.  The glass pans will absorb a little more heat, and so take just a tad longer to heat up - partially balanced by the fact they take a tad longer to cool down, and so might carry over a bit of residual cooking.  Both effects should be pretty minimal, but cookies do bake for a very short amount of time, so the difference may be noticeable in a way that longer-cooking dishes would never notice.  

If you look at your cookies, instead of just pulling them out when the time is up, you should notice if they need a touch more baking (I would guess maybe a minute of extra time, if any)... of course, you should probably be doing this anyway, especially if you want to cook them to your desired style (if you prefer a bit softer or a bit crisper, for example).  Light browning on edges or ridges should be a very good cue for done-ness.