Reading and Writing Documents

The core write functions

getDoc reads one document and returns a snapshot (exists(), data(), id). setDoc creates or overwrites a document; with { merge: true } it merges fields instead. updateDoc changes specific fields and fails if the document does not exist; dotted paths such as "address.city" update nested fields. addDoc adds a document with an auto-generated id. deleteDoc removes one. Use serverTimestamp() instead of the client clock for times, and field transforms such as increment(), arrayUnion() and arrayRemove() for safe concurrent updates.

CRUD with the modular SDK

TypeScript.

import {
  doc, collection, getDoc, setDoc, updateDoc, addDoc, deleteDoc,
  serverTimestamp, increment, arrayUnion,
} from "firebase/firestore";

const profileRef = doc(db, "profiles", uid);

await setDoc(profileRef, { displayName: "Asha", createdAt: serverTimestamp() });
await setDoc(profileRef, { bio: "Builder" }, { merge: true }); // keep other fields
await updateDoc(profileRef, { "address.city": "Pune", loginCount: increment(1) });

const snap = await getDoc(profileRef);
console.log(snap.exists() ? snap.data() : "missing");

const postRef = await addDoc(collection(db, "posts"), {
  authorId: uid, title: "Hello", tags: [], createdAt: serverTimestamp(),
});
await updateDoc(postRef, { tags: arrayUnion("intro") });
await deleteDoc(postRef);

Type your converters

In TypeScript, withConverter on a reference maps snapshots to typed objects, so data() returns your model type instead of DocumentData.

Quick check: What does setDoc do without the merge option when the document already exists?

• Appends a new version
• Fails with not-found
• Overwrites the whole document with the new data
• Merges only changed fields