footballApp-createUserDBEntry.js

// @flow

import firebase from 'react-native-firebase';

export const DB = firebase.firestore();

export const USER_COLLECTION = 'users'; // name of collection in Firestore

export async function createUserDataBaseEntry(userId: string) {
  // creating a new transaction
  return DB.runTransaction(async transaction => {
    // get reference to 'users' collection, pointing to document with name of user's ID
    const userDoc = DB.collection(USER_COLLECTION).doc(userId);

    // create empty user
    transaction.set(userDoc, {
      name: null,
      surname: null,
    });
  }).catch(e => alert('Something went wrong:', e.message));
}