Firebase gives you a complete backend in minutes - auth, database, storage, functions, hosting. But the ease of setup hides real complexity. Security rules are your last line of defense, and they're often wrong.
Firebase gives you a complete backend in minutes - auth, database, storage, functions, hosting. But the ease of setup hides real complexity. Security rules are your last line of defense, and they're often wrong. Firestore queries are limited, and you learn this after you've designed your data model.
This skill covers Firebase Authentication, Firestore, Realtime Database, Cloud Functions, Cloud Storage, and Firebase Hosting. Key insight: Firebase is optimized for read-heavy, denormalized data. If you're thinking relationally, you're thinking wrong.
2025 lesson: Firestore pricing can surprise you. Reads are cheap until they're not. A poorly designed listener can cost more than a dedicated database. Plan your data model for your query patterns, not your data relationships.
Import only what you need for smaller bundles
When to use: Client-side Firebase usage
""" Firebase v9+ uses modular SDK. Import only what you need. This enables tree-shaking and smaller bundles. """
// WRONG: v8-compat style (larger bundle) import firebase from 'firebase/compat/app'; import 'firebase/compat/firestore'; const db = firebase.firestore();
// RIGHT: v9+ modular (tree-shakeable) import { initializeApp } from 'firebase/app'; import { getFirestore, collection, doc, getDoc } from 'firebase/firestore';
const app = initializeApp(firebaseConfig); const db = getFirestore(app);
// Get a document const docRef = doc(db, 'users', 'userId'); const docSnap = await getDoc(docRef);
if (docSnap.exists()) { console.log(docSnap.data()); }
// Query with constraints import { query, where, orderBy, limit } from 'firebase/firestore';
const q = query( collection(db, 'posts'), where('published', '==', true), orderBy('createdAt', 'desc'), limit(10) );
Secure your data with proper rules from day one
When to use: Any Firestore database
""" Rules are your last line of defense. Every read and write goes through them. Get them wrong, and your data is exposed. """
rules_version = '2'; service cloud.firestore { match /databases/{database}/documents {
// Helper functions
function isSignedIn() {
return request.auth != null;
}
function isOwner(userId) {
return request.auth.uid == userId;
}
function isAdmin() {
return request.auth.token.admin == true;
}
// Users collection
match /users/{userId} {
// Anyone can read public profile
allow read: if true;
// Only owner can write their own data
allow write: if isOwner(userId);
// Private subcollection
match /private/{document=**} {
allow read, write: if isOwner(userId);
}
}
// Posts collection
match /posts/{postId} {
// Anyone can read published posts
allow read: if resource.data.published == true
|| isOwner(resource.data.authorId);
// Only authenticated users can create
allow create: if isSignedIn()
&& request.resource.data.authorId == request.auth.uid;
// Only author can update/delete
allow update, delete: if isOwner(resource.data.authorId);
}
// Admin-only collection
match /admin/{document=**} {
allow read, write: if isAdmin();
}
} }
Design Firestore data structure around query patterns
When to use: Designing Firestore schema
""" Firestore is NOT relational. You can't JOIN. Design your data for how you'll QUERY it, not how it relates. """
// WRONG: Normalized (SQL thinking) // users/{userId} // posts/{postId} with authorId field // To get "posts by user" - need to query posts collection
// RIGHT: Denormalized for queries // users/{userId}/posts/{postId} - subcollection // OR // posts/{postId} with embedded author data
// Document structure for a post const post = { id: 'post123', title: 'My Post', content: '...',
// Embed frequently-needed author data author: { id: 'user456', name: 'Jane Doe', avatarUrl: '...' },
// Arrays for IN queries (max 30 items for 'in') tags: ['javascript', 'firebase'],
// Maps for compound queries stats: { likes: 42, comments: 7, views: 1000 },
// Timestamps createdAt: serverTimestamp(), updatedAt: serverTimestamp(),
// Booleans for filtering published: true, featured: false };
// Query patterns this enables: // - Get post with author info: 1 read (no join needed) // - Posts by tag: where('tags', 'array-contains', 'javascript') // - Featured posts: where('featured', '==', true) // - Recent posts: orderBy('createdAt', 'desc')
// When author updates their name, update all their posts // This is the tradeoff: writes are more complex, reads are fast
Subscribe to data changes with proper cleanup
When to use: Real-time features
""" onSnapshot creates a persistent connection. Always unsubscribe when component unmounts to prevent memory leaks and extra reads. """
// React hook for real-time document function useDocument(path) { const [data, setData] = useState(null); const [loading, setLoading] = useState(true); const [error, setError] = useState(null);
useEffect(() => { const docRef = doc(db, path);
// Subscribe to document
const unsubscribe = onSnapshot(
docRef,
(snapshot) => {
if (snapshot.exists()) {
setData({ id: snapshot.id, ...snapshot.data() });
} else {
setData(null);
}
setLoading(false);
},
(err) => {
setError(err);
setLoading(false);
}
);
// Cleanup on unmount
return () => unsubscribe();
}, [path]);
return { data, loading, error }; }
// Usage
function UserProfile({ userId }) {
const { data: user, loading } = useDocument(users/${userId});
if (loading) return <Spinner />; return <div>{user?.name}</div>; }
// Collection with query function usePosts(limit = 10) { const [posts, setPosts] = useState([]);
useEffect(() => { const q = query( collection(db, 'posts'), where('published', '==', true), orderBy('createdAt', 'desc'), limit(limit) );
const unsubscribe = onSnapshot(q, (snapshot) => {
const results = snapshot.docs.map(doc => ({
id: doc.id,
...doc.data()
}));
setPosts(results);
});
return () => unsubscribe();
}, [limit]);
return posts; }
Server-side logic with Cloud Functions v2
When to use: Backend logic, triggers, scheduled tasks
""" Cloud Functions run server-side code triggered by events. V2 uses more standard Node.js patterns and better scaling. """
import { onRequest } from 'firebase-functions/v2/https'; import { onDocumentCreated } from 'firebase-functions/v2/firestore'; import { onSchedule } from 'firebase-functions/v2/scheduler'; import { getFirestore } from 'firebase-admin/firestore'; import { initializeApp } from 'firebase-admin/app';
initializeApp(); const db = getFirestore();
// HTTP function export const api = onRequest( { cors: true, region: 'us-central1' }, async (req, res) => { // Verify auth token const token = req.headers.authorization?.split('Bearer ')[1]; if (!token) { res.status(401).json({ error: 'Unauthorized' }); return; }
try {
const decoded = await getAuth().verifyIdToken(token);
// Process request with decoded.uid
res.json({ userId: decoded.uid });
} catch (error) {
res.status(401).json({ error: 'Invalid token' });
}
} );
// Firestore trigger - on document create export const onUserCreated = onDocumentCreated( 'users/{userId}', async (event) => { const snapshot = event.data; const userId = event.params.userId;
if (!snapshot) return;
const userData = snapshot.data();
// Send welcome email, create related documents, etc.
await db.collection('notifications').add({
userId,
type: 'welcome',
message: `Welcome, ${userData.name}!`,
createdAt: FieldValue.serverTimestamp()
});
} );
// Scheduled function (every day at midnight) export const dailyCleanup = onSchedule( { schedule: '0 0 * * *', timeZone: 'UTC' }, async (event) => { const cutoff = new Date(); cutoff.setDate(cutoff.getDate() - 30);
// Delete old documents
const oldDocs = await db.collection('logs')
.where('createdAt', '<', cutoff)
.limit(500)
.get();
const batch = db.batch();
oldDocs.docs.forEach(doc => batch.delete(doc.ref));
await batch.commit();
console.log(`Deleted ${oldDocs.size} old logs`);
} );
Atomic writes and transactions for consistency
When to use: Multiple document updates that must succeed together
""" Batches: Multiple writes that all succeed or all fail. Transactions: Read-then-write operations with consistency. Max 500 operations per batch/transaction. """
import { writeBatch, runTransaction, doc, getDoc, increment, serverTimestamp } from 'firebase/firestore';
// Batch write - no reads, just writes async function createPostWithTags(post, tags) { const batch = writeBatch(db);
// Create post const postRef = doc(collection(db, 'posts')); batch.set(postRef, { ...post, createdAt: serverTimestamp() });
// Update tag counts for (const tag of tags) { const tagRef = doc(db, 'tags', tag); batch.set(tagRef, { count: increment(1), lastUsed: serverTimestamp() }, { merge: true }); }
await batch.commit(); return postRef.id; }
// Transaction - read and write atomically async function likePost(postId, userId) { return runTransaction(db, async (transaction) => { const postRef = doc(db, 'posts', postId); const likeRef = doc(db, 'posts', postId, 'likes', userId);
const postSnap = await transaction.get(postRef);
if (!postSnap.exists()) {
throw new Error('Post not found');
}
const likeSnap = await transaction.get(likeRef);
if (likeSnap.exists()) {
throw new Error('Already liked');
}
// Increment like count and add like document
transaction.update(postRef, {
likeCount: increment(1)
});
transaction.set(likeRef, {
userId,
createdAt: serverTimestamp()
});
return postSnap.data().likeCount + 1;
}); }
OAuth provider setup and authentication flows
When to use: Social login implementation
import { getAuth, signInWithPopup, signInWithRedirect, GoogleAuthProvider, GithubAuthProvider, OAuthProvider } from "firebase/auth";
const auth = getAuth();
// GOOGLE const googleProvider = new GoogleAuthProvider(); googleProvider.addScope("email"); googleProvider.setCustomParameters({ prompt: "select_account" });
async function signInWithGoogle() { try { const result = await signInWithPopup(auth, googleProvider); return result.user; } catch (error) { if (error.code === "auth/account-exists-with-different-credential") { return handleAccountConflict(error); } throw error; } }
// GITHUB const githubProvider = new GithubAuthProvider(); githubProvider.addScope("read:user");
// APPLE (Required for iOS apps!) const appleProvider = new OAuthProvider("apple.com"); appleProvider.addScope("email"); appleProvider.addScope("name");
When to use popup vs redirect for OAuth
When to use: Choosing authentication flow
async function signIn(provider) { if (/iPhone|iPad|Android/i.test(navigator.userAgent)) { return signInWithRedirect(auth, provider); } try { return await signInWithPopup(auth, provider); } catch (e) { if (e.code === "auth/popup-blocked") { return signInWithRedirect(auth, provider); } throw e; } }
// Check redirect result on page load useEffect(() => { getRedirectResult(auth).then(r => r && setUser(r.user)); }, []);
Link multiple providers to one account
When to use: User has accounts with different providers
import { fetchSignInMethodsForEmail, linkWithCredential } from "firebase/auth";
async function handleAccountConflict(error) { const email = error.customData?.email; const pendingCred = OAuthProvider.credentialFromError(error); const methods = await fetchSignInMethodsForEmail(auth, email);
if (methods.includes("google.com")) { alert("Sign in with Google to link accounts"); const result = await signInWithPopup(auth, new GoogleAuthProvider()); await linkWithCredential(result.user, pendingCred); return result.user; } }
// Link new provider await linkWithPopup(auth.currentUser, new GithubAuthProvider());
// Unlink provider (keep at least one!) await unlink(auth.currentUser, "github.com");
Control session lifetime
When to use: Managing user sessions
import { setPersistence, browserLocalPersistence, browserSessionPersistence } from "firebase/auth";
// LOCAL: survives browser close (default) // SESSION: cleared on tab close
async function signInWithRememberMe(email, pass, remember) { await setPersistence(auth, remember ? browserLocalPersistence : browserSessionPersistence); return signInWithEmailAndPassword(auth, email, pass); }
// React auth hook function useAuth() { const [user, setUser] = useState(null); const [loading, setLoading] = useState(true); useEffect(() => onAuthStateChanged(auth, u => { setUser(u); setLoading(false); }), []); return { user, loading }; }
Complete email auth flow
When to use: Email/password authentication
import { sendEmailVerification, sendPasswordResetEmail, reauthenticateWithCredential } from "firebase/auth";
// Sign up with verification async function signUp(email, password) { const result = await createUserWithEmailAndPassword(auth, email, password); await sendEmailVerification(result.user); return result.user; }
// Password reset await sendPasswordResetEmail(auth, email);
// Change password (requires recent auth) const cred = EmailAuthProvider.credential(user.email, currentPass); await reauthenticateWithCredential(user, cred); await updatePassword(user, newPass);
Handle ID tokens for backend calls
When to use: Authenticating with backend APIs
import { getIdToken, onIdTokenChanged } from "firebase/auth";
// Get token (auto-refreshes if expired) const token = await getIdToken(auth.currentUser);
// API helper with auto-retry async function apiCall(url, opts = {}) { const token = await getIdToken(auth.currentUser); const res = await fetch(url, { ...opts, headers: { ...opts.headers, Authorization: "Bearer " + token } }); if (res.status === 401) { const newToken = await getIdToken(auth.currentUser, true); return fetch(url, { ...opts, headers: { ...opts.headers, Authorization: "Bearer " + newToken }}); } return res; }
// Sync to cookie for SSR onIdTokenChanged(auth, async u => { document.cookie = u ? "__session=" + await u.getIdToken() : "__session=; max-age=0"; });
// Check admin claim const { claims } = await auth.currentUser.getIdTokenResult(); const isAdmin = claims.admin === true;
Works well with: nextjs-app-router, react-patterns, authentication-oauth, stripe