firebase

SKILL.md

Firebase

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

Capabilities

firebase-auth

firestore

firebase-realtime-database

firebase-cloud-functions

firebase-storage

firebase-hosting

firebase-security-rules

firebase-admin-sdk

firebase-emulators

Scope

general-backend-architecture -> backend

payment-processing -> stripe

email-sending -> email

advanced-auth-flows -> authentication-oauth

kubernetes-deployment -> devops

Tooling

Core

firebase - When: Client-side SDK Note: Modular SDK - tree-shakeable

firebase-admin - When: Server-side / Cloud Functions Note: Full access, bypasses security rules

firebase-functions - When: Cloud Functions v2 Note: v2 functions are recommended

Testing

@firebase/rules-unit-testing - When: Testing security rules Note: Essential - rules bugs are security bugs

firebase-tools - When: Emulator suite Note: Local development without hitting production

Frameworks

reactfire - When: React + Firebase Note: Hooks-based, handles subscriptions

vuefire - When: Vue + Firebase Note: Vue-specific bindings

angularfire - When: Angular + Firebase Note: Official Angular bindings

Patterns

Modular SDK Import

Import only what you need for smaller bundles

When to use: Client-side Firebase usage

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)
);

Security Rules Design

Secure your data with proper rules from day one

When to use: Any Firestore database

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();
}

}
}

Data Modeling for Queries

Design Firestore data structure around query patterns

When to use: Designing Firestore schema

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

Real-time Listeners

Subscribe to data changes with proper cleanup

When to use: Real-time features

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 ;
return {user?.name};
}

// 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;
}

Cloud Functions Patterns

Server-side logic with Cloud Functions v2

When to use: Backend logic, triggers, scheduled tasks

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`);

}
);

Batch Operations

Atomic writes and transactions for consistency

When to use: Multiple document updates that must succeed together

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;

});
}

Social Login (Google, GitHub, etc.)

OAuth provider setup and authentication flows

When to use: Social login implementation

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");

Popup vs Redirect Auth

When to use popup vs redirect for OAuth

When to use: Choosing authentication flow

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));
}, []);

Account Linking

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");

Auth State Persistence

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 };
}

Email Verification and Password Reset

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);

Token Management for APIs

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;

Collaboration

Delegation Triggers

user needs complex OAuth flow -> authentication-oauth (Firebase Auth handles basics, complex flows need OAuth skill)

user needs payment integration -> stripe (Firebase + Stripe common pattern)

user needs email functionality -> email (Firebase doesn't include email - use SendGrid, Resend, etc.)

user needs container deployment -> devops (Beyond Firebase Hosting - Kubernetes, Docker)

user needs relational data model -> postgres-wizard (Firestore is wrong choice for highly relational data)

user needs full-text search -> elasticsearch-search (Firestore doesn't support full-text search - use Algolia/Elastic)

Related Skills

Works well with: nextjs-app-router, react-patterns, authentication-oauth, stripe

When to Use

User mentions or implies: firebase

User mentions or implies: firestore

User mentions or implies: firebase auth

User mentions or implies: cloud functions

User mentions or implies: firebase storage

User mentions or implies: realtime database

User mentions or implies: firebase hosting

User mentions or implies: firebase emulator

User mentions or implies: security rules

User mentions or implies: firebase admin

Limitations

Use this skill only when the task clearly matches the scope described above.

Do not treat the output as a substitute for environment-specific validation, testing, or expert review.

Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.