How to Use Firestore Triggers in Firebase Cloud Functions

1firebase init functions
2# Choose TypeScript, select your project
3# Verify engines in functions/package.json:
4# "engines": { "node": "20" }

1import { onDocumentCreated } from "firebase-functions/v2/firestore";
2import { logger } from "firebase-functions";
3
4export const onOrderCreated = onDocumentCreated(
5  "orders/{orderId}",
6  async (event) => {
7    const snapshot = event.data;
8    if (!snapshot) {
9      logger.warn("No data in event");
10      return;
11    }
12
13    const orderData = snapshot.data();
14    const orderId = event.params.orderId;
15
16    logger.info(`New order ${orderId}:`, orderData);
17
18    // Example: send a notification, update inventory, etc.
19  }
20);

1import { onDocumentUpdated } from "firebase-functions/v2/firestore";
2import { logger } from "firebase-functions";
3
4export const onOrderStatusChanged = onDocumentUpdated(
5  "orders/{orderId}",
6  async (event) => {
7    if (!event.data) return;
8
9    const before = event.data.before.data();
10    const after = event.data.after.data();
11
12    // Only act if the status field actually changed
13    if (before.status === after.status) {
14      return;
15    }
16
17    logger.info(
18      `Order ${event.params.orderId} status: ${before.status} → ${after.status}`
19    );
20
21    if (after.status === "shipped") {
22      // Send shipping notification to customer
23    }
24  }
25);

1import { onDocumentDeleted } from "firebase-functions/v2/firestore";
2import { getFirestore } from "firebase-admin/firestore";
3import * as admin from "firebase-admin";
4
5admin.initializeApp();
6const db = getFirestore();
7
8export const onUserDeleted = onDocumentDeleted(
9  "users/{userId}",
10  async (event) => {
11    if (!event.data) return;
12
13    const userId = event.params.userId;
14
15    // Clean up the user's posts
16    const postsSnap = await db
17      .collection("posts")
18      .where("authorId", "==", userId)
19      .get();
20
21    const batch = db.batch();
22    postsSnap.docs.forEach((doc) => batch.delete(doc.ref));
23    await batch.commit();
24  }
25);

1import { onDocumentCreatedWithAuthContext } from "firebase-functions/v2/firestore";
2import { logger } from "firebase-functions";
3
4export const auditOrderCreation = onDocumentCreatedWithAuthContext(
5  "orders/{orderId}",
6  async (event) => {
7    const userId = event.authId;    // UID of the user who wrote the doc
8    const authType = event.authType; // "admin" | "user" | "unauthenticated"
9
10    logger.info(
11      `Order ${event.params.orderId} created by ${userId} (${authType})`
12    );
13  }
14);

1export const enrichOrder = onDocumentCreated(
2  "orders/{orderId}",
3  async (event) => {
4    if (!event.data) return;
5
6    const data = event.data.data();
7
8    // Guard: skip if already processed by this function
9    if (data.enrichedByFunction === true) {
10      return;
11    }
12
13    // Safe to write back — the guard prevents re-triggering
14    await event.data.ref.update({
15      enrichedByFunction: true,
16      totalWithTax: data.subtotal * 1.1,
17      processedAt: admin.firestore.FieldValue.serverTimestamp(),
18    });
19  }
20);

1# Deploy only functions (not hosting, rules, etc.)
2firebase deploy --only functions
3
4# Deploy a specific function
5firebase deploy --only functions:onOrderCreated
6
7# View logs after triggering
8firebase functions:log --only onOrderCreated