how to get the user uid from firebase email authentication code example
Example: firebase auth get current user
var user = firebase.auth().currentUser;var name, email, photoUrl, uid, emailVerified;if (user != null) { name = user.displayName; email = user.email; photoUrl = user.photoURL; emailVerified = user.emailVerified; uid = user.uid; // The user's ID, unique to the Firebase project. Do NOT use // this value to authenticate with your backend server, if // you have one. Use User.getToken() instead.}