Firebase: How to get User.uid?

You might want to use a guard, as a force unwrap might break your app if a user isn’t signed in.

guard let userID = Auth.auth().currentUser?.uid else { return }

This is how you get the user's uid:

let userID = Auth.auth().currentUser!.uid

UPDATE: Since this answer is getting upvotes, make sure you prevent your app from crashing by using guards:

guard let userID = Auth.auth().currentUser?.uid else { return }