Electrical – Is it safe to put a microwave in a walk-in closet

All dryers need a vent. Gas and electric dryers need a vent to the outside of the house to allow all the moisture to escape from the dryer.

All-in-one washer/dryer combo units don't need an air vent, but need a water drain to remove the water from the drying stage (which you need anyways to drain the wash cycle).

The walls are irrelevant. So don't worry about those.

The bigger concern would be the floor. If this is a high efficiency front-loading washer, it will be prone to lots of lateral shaking. Most wood framed floors are ill equipped to handle the speed of front loader washers. It's usually recommended that they be placed on concrete slabs.

As such if the closet wasn't designed for a washer/dryer, then you probably need to beef up the floor as much as you can to reduce the shaking.

Microwaves go in the kitchen. I would suggest that it is up in a higher spot so kids can't play with them. That is a good reason they are often over ovens along with the venting.

Also from an electric point of view almost all microwaves require a 12A dedicated circuit. For a kitchen you will probably have several runs like this and it is just bundled with those. For a bedroom you would be basically running one circuit into a room for one outlet, where the rest of the room is on another. Just a little odd.

From a safety stand-point microwaves draw a lot of current. I guess there is more of a chance (albeit very low) that microwaves cause more fires due to current fluctuations than most other things. I would much rather a fire in my kitchen than my bedroom or anyone's bedroom.

And practically speaking I remember living in the dorm and my roommate burning popcorn in the microwave... Good luck falling asleep with burnt popcorn smell.