deleteUserById.js

// Deletes a user by ID in firestore user collection & firebase auth
static async deleteUserById(id) {
  const userRef = firestore
    .collection(this.collectionName())
    .doc(id);

  const batch = firestore.batch();
  batch.delete(userRef);

  const completedTasksToDelete = await firestore
    .collection('completedTasks')
    .where('userId', '==', id)
    .get();
  const savedTasksToDelete = await firestore
    .collection('savedTasks')
    .where('userId', '==', id)
    .get();

  completedTasksToDelete.docs.forEach(doc => batch.delete(doc.ref));
  savedTasksToDelete.docs.forEach(doc => batch.delete(doc.ref));

  try {
    // Delete the user record and all other records related to it
    await batch.commit();
    // Delete the user from the auth table
    await admin.auth().deleteUser(id);
  } catch (e) {
    throw new Error('Could not delete user.');
  }
}