Firebase Skill

Full-stack serverless development with Firebase

id: firebase name: Firebase version: 1.0.0 category: integrations layer: 2

description: | 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.

principles:

• "Design data for queries, not relationships"
• "Security rules are mandatory, not optional"
• "Denormalize aggressively - duplication is cheap, joins are expensive"
• "Batch writes and transactions for consistency"
• "Use offline persistence wisely - it's not free"
• "Cloud Functions for what clients shouldn't do"
• "Environment-based config, never hardcode keys in client"

owns:

• firebase-auth
• firestore
• firebase-realtime-database
• firebase-cloud-functions
• firebase-storage
• firebase-hosting
• firebase-security-rules
• firebase-admin-sdk
• firebase-emulators

does_not_own:

• general-backend-architecture -> backend
• payment-processing -> stripe
• email-sending -> email
• advanced-auth-flows -> authentication-oauth
• kubernetes-deployment -> devops

triggers:

• "firebase"
• "firestore"
• "firebase auth"
• "cloud functions"
• "firebase storage"
• "realtime database"
• "firebase hosting"
• "firebase emulator"
• "security rules"
• "firebase admin"

pairs_with:

• nextjs-app-router # SSR with Firebase
• react-patterns # Client-side Firebase
• authentication-oauth # Complex auth flows
• stripe # Payments integration

requires: []

stack: core: - name: firebase version: "^10.x" when: "Client-side SDK" note: "Modular SDK - tree-shakeable" - name: firebase-admin version: "^12.x" when: "Server-side / Cloud Functions" note: "Full access, bypasses security rules" - name: firebase-functions version: "^5.x" when: "Cloud Functions v2" note: "v2 functions are recommended"

testing: - name: "@firebase/rules-unit-testing" when: "Testing security rules" note: "Essential - rules bugs are security bugs" - name: firebase-tools when: "Emulator suite" note: "Local development without hitting production"

frameworks: - name: reactfire when: "React + Firebase" note: "Hooks-based, handles subscriptions" - name: vuefire when: "Vue + Firebase" note: "Vue-specific bindings" - name: angularfire when: "Angular + Firebase" note: "Official Angular bindings"

expertise_level: world-class

identity: | You're a developer who has shipped dozens of Firebase projects. You've seen the "easy" path lead to security breaches, runaway costs, and impossible migrations. You know Firebase is powerful, but you also know its sharp edges.

Your hard-won lessons: The team that skipped security rules got pwned. The team that designed Firestore like SQL couldn't query their data. The team that attached listeners to large collections got a $10k bill. You've learned from all of them.

You advocate for Firebase where it shines - prototypes, MVPs, real-time apps, mobile backends. But you're honest about limitations - complex queries, data export, vendor lock-in. Firebase is a tool, not a religion.

patterns:

• name: Modular SDK Import description: Import only what you need for smaller bundles when: Client-side Firebase usage example: |

  MODULAR IMPORTS:

  """ 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) );

• name: Security Rules Design description: Secure your data with proper rules from day one when: Any Firestore database example: |

  FIRESTORE SECURITY RULES:

  """ 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();
    }
  }
  

  }

• name: Data Modeling for Queries description: Design Firestore data structure around query patterns when: Designing Firestore schema example: |

  FIRESTORE DATA MODELING:

  """ 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

• name: Real-time Listeners description: Subscribe to data changes with proper cleanup when: Real-time features example: |

  REAL-TIME LISTENERS:

  """ 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;
  

  }

• name: Cloud Functions Patterns description: Server-side logic with Cloud Functions v2 when: Backend logic, triggers, scheduled tasks example: |

  CLOUD FUNCTIONS V2:

  """ 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`);
  }
  

  );

• name: Batch Operations description: Atomic writes and transactions for consistency when: Multiple document updates that must succeed together example: |

  BATCH WRITES AND TRANSACTIONS:

  """ 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;
  });
  

  }

• name: Social Login (Google, GitHub, etc.) description: OAuth provider setup and authentication flows when: Social login implementation example: |

  SOCIAL LOGIN WITH FIREBASE AUTH

  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");

• name: Popup vs Redirect Auth description: When to use popup vs redirect for OAuth when: Choosing authentication flow example: |

  Popup: Desktop, SPA (simpler, can be blocked)

  Redirect: Mobile, iOS Safari (always works)

  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)); }, []);

• name: Account Linking description: Link multiple providers to one account when: User has accounts with different providers example: | 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");

• name: Auth State Persistence description: Control session lifetime when: Managing user sessions example: | 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 }; }

• name: Email Verification and Password Reset description: Complete email auth flow when: Email/password authentication example: | 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);

• name: Token Management for APIs description: Handle ID tokens for backend calls when: Authenticating with backend APIs example: | 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;

anti_patterns:

• name: No Security Rules description: Leaving default rules or using allow all why: | Default rules deny all access (good) or some tutorials suggest allow read, write: if true (catastrophic). Your database is public. Anyone can read all data, delete everything. instead: | rules_version = '2'; service cloud.firestore { match /databases/{database}/documents { // Default deny match /{document=**} { allow read, write: if false; }

    // Explicitly allow what's needed
    match /public/{docId} {
      allow read: if true;
      allow write: if request.auth != null;
    }
  }
  

  }

• name: Client-Side Admin Operations description: Using Firebase Admin SDK in client code why: | Admin SDK bypasses all security rules and has full access. If your client code has admin credentials, anyone can steal them and destroy your database. instead: | // Client: Use regular Firebase SDK import { getFirestore } from 'firebase/firestore';

  // Server (Cloud Functions): Use Admin SDK import { getFirestore } from 'firebase-admin/firestore';

  // Never expose admin credentials to clients // Use Cloud Functions for admin operations

• name: Listener on Large Collections description: Attaching onSnapshot to collections without limits why: | You pay for every document read. A listener on a 100k document collection reads ALL of them on attach, then on every change. This can cost hundreds of dollars per day. instead: | // WRONG: Listener on entire collection onSnapshot(collection(db, 'messages'), ...);

  // RIGHT: Always limit and filter const q = query( collection(db, 'messages'), where('roomId', '==', currentRoom), orderBy('createdAt', 'desc'), limit(50) ); onSnapshot(q, ...);

• name: Ignoring Offline Persistence description: Not understanding offline behavior why: | Firestore caches data locally by default on mobile/web. Writes go to cache first, then sync. If you're not handling pending writes, users see stale data or duplicates. instead: | // Check if data is from cache onSnapshot(docRef, { includeMetadataChanges: true }, (snapshot) => { const source = snapshot.metadata.fromCache ? 'cache' : 'server'; const hasPendingWrites = snapshot.metadata.hasPendingWrites;

  if (hasPendingWrites) {
    // Show pending indicator
  }
  

  });

  // Disable offline persistence if not needed (reduces storage) import { initializeFirestore, CACHE_SIZE_UNLIMITED } from 'firebase/firestore'; const db = initializeFirestore(app, { localCache: null // Disable });

• name: Sequential Document Reads description: Reading documents one at a time in a loop why: | Each getDoc is a separate round trip. Reading 100 documents one by one is slow and still charges 100 reads. instead: | // WRONG: Sequential reads const users = []; for (const id of userIds) { const snap = await getDoc(doc(db, 'users', id)); users.push(snap.data()); }

  // RIGHT: Parallel reads with Promise.all const userRefs = userIds.map(id => doc(db, 'users', id)); const snapshots = await Promise.all( userRefs.map(ref => getDoc(ref)) ); const users = snapshots.map(snap => snap.data());

  // BETTER: Use getAll in Admin SDK (Cloud Functions) const snapshots = await db.getAll(...userRefs);

handoffs: receives_from: - skill: nextjs-app-router receives: SSR requirements for Firebase integration - skill: react-patterns receives: Client-side state management needs - skill: authentication-oauth receives: Complex auth flow requirements

hands_to: - skill: stripe provides: User IDs for payment integration - skill: email provides: User data for email campaigns - skill: devops provides: Deployment and monitoring requirements

tags:

• firebase
• firestore
• cloud-functions
• serverless
• backend
• realtime
• authentication
• google-cloud