Cost optimization (read/write reduction) in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to read data from Firestore only once.

Firebase

const docRef = firestore.collection('users').doc('user1');
const doc = await docRef.[1]();

Drag options to blanks, or click blank then click option'

Aget

BonSnapshot

Cset

Dupdate

Attempts:

3 left

💡 Hint

Common Mistakes

Using onSnapshot causes continuous reads and higher costs.

Using set or update is for writing, not reading.

Using get() reads the document once, reducing read costs compared to onSnapshot() which listens continuously.

2fill in blank

medium

Complete the code to batch write multiple documents to reduce write operations.

Firebase

const batch = firestore.batch();
const doc1 = firestore.collection('orders').doc('order1');
batch.set(doc1, {status: 'pending'});
const doc2 = firestore.collection('orders').doc('order2');
batch.[1](doc2, {status: 'pending'});
await batch.commit();

Drag options to blanks, or click blank then click option'

Aset

Bupdate

Cdelete

Dget

Attempts:

3 left

💡 Hint

Common Mistakes

Using get is for reading, not writing.

Using delete removes documents instead of writing.

Using set() in batch writes multiple documents efficiently, reducing the number of separate write operations.

3fill in blank

hard

Fix the error in the Firestore query to limit reads and reduce costs.

Firebase

const querySnapshot = await firestore.collection('products').[1]().limit(10).get();

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Awhere('price', '>', 100)

Bget

CorderBy('price')

Dlimit

Attempts:

3 left

💡 Hint

Common Mistakes

Using get() here is incorrect because it is a method to execute the query, not build it.

Using limit() inside the chain twice causes errors.

Using orderBy('price') before limit(10) ensures the query returns the first 10 products sorted by price, reducing unnecessary reads.

4fill in blank

hard

Fill both blanks to create a Firestore query that fetches only active users with minimal reads.

Firebase

const activeUsersQuery = firestore.collection('users').[1]('status', '==', [2]);

Drag options to blanks, or click blank then click option'

Awhere

B'active'

C'inactive'

DorderBy

Attempts:

3 left

💡 Hint

Common Mistakes

Using orderBy instead of where does not filter results.

Using 'inactive' fetches unwanted users, increasing reads.

Using where('status', '==', 'active') filters users to only those active, reducing the number of documents read.

5fill in blank

hard

Fill all three blanks to create a Firestore listener that unsubscribes after first data fetch to reduce continuous reads.

Firebase

const unsubscribe = firestore.collection('messages').onSnapshot(snapshot => {
  snapshot.docs.forEach(doc => console.log(doc.data()));
  [1]();
});

// Later in code
[2] = unsubscribe;
[3]();

Drag options to blanks, or click blank then click option'

Aunsubscribe

Bconst stopListening

CstopListening

Dsubscribe

Attempts:

3 left

💡 Hint

Common Mistakes

Calling subscribe() is incorrect; onSnapshot already subscribes.

Not calling unsubscribe causes continuous reads and higher costs.

Calling unsubscribe() inside the listener stops further reads. Assigning it to a variable and calling that variable later also stops listening, reducing costs.