Firestore CRUD operations in React Native - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What happens when this Firestore document is deleted?

Consider this React Native code snippet that deletes a Firestore document. What will be the result in the app UI after the deletion succeeds?

React Native

import { doc, deleteDoc } from 'firebase/firestore';
import { db } from './firebaseConfig';

async function deleteUserDoc(userId) {
  const docRef = doc(db, 'users', userId);
  await deleteDoc(docRef);
  console.log('User deleted');
}

// Called on button press: deleteUserDoc('abc123');

A Nothing happens because deleteDoc only marks the document for deletion but does not remove it.

B The user document is marked as deleted but still visible in the UI until app restart.

C The document is copied to a backup collection before deletion, so UI shows both original and backup.

D The user document with ID 'abc123' is removed from Firestore and the UI updates to no longer show that user.

💡 Hint

Deleting a document removes it immediately from Firestore and listeners update the UI.

The deleteDoc function removes the document from Firestore immediately. If the UI listens to Firestore changes, it will update to reflect the deletion.

📝 Syntax

intermediate

Which option correctly adds a new Firestore document with auto-generated ID?

You want to add a new document to the 'tasks' collection with a field 'title' set to 'Buy milk'. Which code snippet does this correctly?

A

import { collection, setDoc } from 'firebase/firestore';
const tasksRef = collection(db, 'tasks');
await setDoc(tasksRef, { title: 'Buy milk' });

B

import { collection, addDoc } from 'firebase/firestore';
const tasksRef = collection(db, 'tasks');
await addDoc(tasksRef, { title: 'Buy milk' });

C

import { doc, setDoc } from 'firebase/firestore';
const tasksRef = doc(db, 'tasks');
await setDoc(tasksRef, { title: 'Buy milk' });

D

import { addDoc } from 'firebase/firestore';
await addDoc('tasks', { title: 'Buy milk' });

💡 Hint

Use addDoc with a collection reference to add with auto ID.

addDoc requires a collection reference and data object. It creates a new document with an auto-generated ID.

❓ lifecycle

advanced

What is the effect of this Firestore listener in a React Native component?

This code sets up a Firestore listener inside a React Native component. What happens when the component unmounts?

React Native

import { useEffect, useState } from 'react';
import { collection, onSnapshot } from 'firebase/firestore';
import { db } from './firebaseConfig';

function TaskList() {
  const [tasks, setTasks] = useState([]);

  useEffect(() => {
    const unsubscribe = onSnapshot(collection(db, 'tasks'), (snapshot) => {
      setTasks(snapshot.docs.map(doc => ({ id: doc.id, ...doc.data() })));
    });
    return () => unsubscribe();
  }, []);

  return null;
}

A The listener is removed when the component unmounts, preventing memory leaks.

B The listener continues running even after unmount, causing duplicate updates.

C The listener throws an error on unmount because unsubscribe is not a function.

D The listener pauses but does not unsubscribe, so it resumes on remount.

💡 Hint

Returning unsubscribe from useEffect cleans up the listener.

The unsubscribe function returned by onSnapshot is called on unmount to stop listening and free resources.

🔧 Debug

advanced

Why does this Firestore update code fail with a runtime error?

This code tries to update a Firestore document but throws an error. What is the cause?

React Native

import { doc, updateDoc } from 'firebase/firestore';
import { db } from './firebaseConfig';

async function updateUser(userId) {
  const userRef = doc(db, 'users', userId);
  await updateDoc(userRef, { age: 30 });
}

updateUser('nonexistentId');

A updateDoc fails because the document with 'nonexistentId' does not exist in Firestore.

B updateDoc fails because the 'age' field is missing in the document.

C updateDoc fails because doc() requires a fourth argument specifying the collection.

D updateDoc fails because updateDoc only works with setDoc, not doc references.

💡 Hint

updateDoc requires the document to exist before updating.

updateDoc throws an error if the document does not exist. You must create it first or use setDoc with merge.

🧠 Conceptual

expert

How does Firestore handle offline writes in React Native apps?

When a React Native app writes data to Firestore while offline, what happens to the data and UI?

A Writes are queued on the server and only appear in the UI after syncing.

B Writes fail immediately and show an error in the UI until connection is restored.

C Writes are saved locally and immediately reflected in the UI; they sync to Firestore when online.

D Writes are lost if offline; the app must retry manually when online.

💡 Hint

Firestore supports offline persistence that updates UI instantly.

Firestore caches writes locally and updates the UI immediately. When the device reconnects, it syncs changes to the server.