What is Firebase SDK initialization?

Firebase SDK initialization configures your app to connect to your Firebase project. In Firebase SDK v9 (modular API): import { initializeApp } from "firebase/app"; const firebaseConfig = { apiKey: "your-api-key", authDomain: "project.firebaseapp.com", projectId: "project-id", storageBucket: "project.appspot.com", messagingSenderId: "123456789", appId: "1:123:web:abc" }; const app = initializeApp(firebaseConfig). Then get service instances: import { getFirestore } from "firebase/firestore"; const db = getFirestore(app). Important: (1) The config object is safe to include in client code — Firebase API keys are identifiers, not secrets. Access control is enforced via Security Rules; (2) Call initializeApp() only once (typically in a dedicated firebase.js module); (3) For the Emulator Suite, connect to local emulators: connectFirestoreEmulator(db, "localhost", 8080) for local development.