where does "db" come from?

[carcinocron]

Almost all the docs do this, I have no idea where "db" came from. I'm also not sure if I should be using web9 or nodejs, but it's often a lot of samples are missing one or both, or are less clearly labeled.

snippets-web/snippets/firestore-next/test-firestore/transaction.js

Lines 8 to 23 in 1c4c683

	import { runTransaction } from "firebase/firestore";
	try {
	await runTransaction(db, async (transaction) => {
	const sfDoc = await transaction.get(sfDocRef);
	if (!sfDoc.exists()) {
	throw "Document does not exist!";
	}
	const newPopulation = sfDoc.data().population + 1;
	transaction.update(sfDocRef, { population: newPopulation });
	});
	console.log("Transaction successfully committed!");
	} catch (e) {
	console.log("Transaction failed: ", e);
	}

[Tim4497]

const firebaseConfig = {
  apiKey: 
  authDomain: 
  projectId: 
  storageBucket: 
  messagingSenderId: 
  appId: 
};

const app = initializeApp(firebaseConfig);

const db = getFirestore(app);