Firebase Firestore Cheat Sheet

Firestore's modular JS SDK for reading, writing, querying, and listening to real-time document data in a NoSQL collection model.

2 PagesIntermediateMar 18, 2026

SDK Basics (v9 modular)

Initializing Firestore and writing documents.

javascript

import { initializeApp } from "firebase/app";import { getFirestore, collection, doc, setDoc, addDoc } from "firebase/firestore";const app = initializeApp(firebaseConfig);const db = getFirestore(app);// Add a document with an auto-generated IDawait addDoc(collection(db, "users"), { email: "a@b.com" });// Set a document with a known IDawait setDoc(doc(db, "users", "u1"), { email: "a@b.com" });

Queries & Reads

Filtering, ordering, and limiting results.

javascript

import { query, where, orderBy, limit, getDocs, collection } from "firebase/firestore";const q = query(  collection(db, "users"),  where("age", ">=", 18),  orderBy("age"),  limit(10));const snapshot = await getDocs(q);snapshot.forEach(docSnap => console.log(docSnap.id, docSnap.data()));

Real-Time Listeners & Updates

Subscribing to changes and modifying data.

javascript

import { onSnapshot, updateDoc, deleteDoc, doc } from "firebase/firestore";const unsubscribe = onSnapshot(doc(db, "users", "u1"), (snap) => {  console.log(snap.data());});await updateDoc(doc(db, "users", "u1"), { email: "x@y.com" });await deleteDoc(doc(db, "users", "u1"));

Data Model & Rules

How Firestore organizes and secures data.

Collection- a named group of documents, e.g. "users"
Document- a JSON-like record with fields, max 1MB, stored in a collection
Subcollection- a collection nested inside a document
Security Rules- declarative rules controlling read/write access per document path
Composite index- required for queries that filter or sort on multiple fields
Offline persistence- the SDK caches data locally and syncs automatically on reconnect

Pro Tip

Firestore bills per document read/write/delete, not per query complexity — model your data so common queries touch as few documents as possible rather than optimizing the query syntax itself.

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