How to retrieve uid from a user on firebase with method createUserWithEmailAndPassword

I'm new to firebase, I'm following a tutorial in which I design a firebase chat for IOS with swift, I'm embedding it in my web project, but I've seen that they retrieve the uid with the createUserWithEmailAndPassword method, I see in the documentation that the promise returns a user, but I do not know how to access their properties or how to retrieve it, I would like to know how to access that data, thanks

the code of swift is this

FIRAuth.auth()?createUser(withEmail emai, password:password, completion: user:FIRUser)...