[React] - Firestore 세팅

After v9 :

# import firebase

// v9 compat packages are API compatible with v8 code
import firebase from 'firebase/compat/app';
import 'firebase/compat/auth';
import 'firebase/compat/firestore';

# initailzation

import { initializeApp } from "firebase/app"

const firebaseApp = initializeApp({ /* config */ });

# getAuth

import { getAuth, onAuthStateChanged } from "firebase/auth";

const auth = getAuth(firebaseApp);
onAuthStateChanged(auth, user => {
  // Check for user status
});

# getData & query

import { getFirestore, collection, query, where, getDocs } from "firebase/firestore";

const db = getFirestore(firebaseApp);

const q = query(collection(db, "cities"), where("capital", "==", true));

const querySnapshot = await getDocs(q);
querySnapshot.forEach((doc) => {
  // doc.data() is never undefined for query doc snapshots
  console.log(doc.id, " => ", doc.data());
});

# exist

if (snapshot.exists()) {
  console.log("the document exists");
}

[출처] : Firebase Docs