firebaseCheckExists.js

// where `db` is a `Firebase` ref

export function checkIfUserExists(authData) {
  return db
    .child('users')
    .child(authData.uid)
    .once('value')
    .then(dataSnapshot => {
      return Promise.resolve({
        authData,
        userExists: dataSnapshot.exists(),
      });
    });
}

// example usage
db.authWithOAuthPopup(provider)
  .then(checkIfUserExists)
  .then(({authData, userExists}) => {
    if (userExists) {
      // update user
    } else {
      // go create a user
    }
  })
  .catch(err => {
    console.warn('Error signing in.', err);
  });