Yes WD 40 can clean windows and is especially good for cleaning glass shower doors because it helps to clean the water deposits.
The WD 40 helps to displace water too so the water will run off the glass shower doors when you take a shower.
If you use the WD 40 on your windows to clean them you should go over the windows with some cleaner that gets rid of grease and remove the WD 40 residue otherwise it might leave streaks on your windows.
Or you can just use some vinegar and water clean your windows naturally without leaving any residue or streaks behind.
I use vinegar and water as a natural window cleaner a lot and it works great.
I wash my house windows, mirrors, car windows and windshields using vinegar and water and it does a wonderful job at getting the glass clean.