Set user identifier and UUID in Firebase

If you sign in with a custom token in Firebase, you'll notice that the identifier field of a user is left blank.

You can change the identifier field using updateEmail() function.

Example:

javascript
let { user, additionalUserInfo } = await firebase.auth().signInWithCustomToken(token);

await user.updateEmail('jeff.bezos@amazon.com');

Now Jeff Bezos will have a User UID (UUID) generated by signInWithCustomToken() function and an email as identifier.