Just leaving a note here. If you’re trying to use Multer and you keep getting an undefined value for req.file - try closing Glitch and reopening your project. I’m not sure why that worked but the suggesting came from this thread at stackoverflow.
"This may fix the problem. Every time you restart the server you need to do above steps for calling the API again. The reason being multer sends back some cookies called connect.sid to the client which it may require in further communication. Using old cookies will not upload the file."