Getting a single document in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

What is the output of this Firebase Firestore code snippet?

Consider the following Firebase Firestore code to get a single document by ID:

const docRef = firestore.collection('users').doc('user123');
const docSnap = await docRef.get();
if (docSnap.exists) {
  console.log(docSnap.data());
} else {
  console.log('No such document!');
}

What will be printed if the document with ID 'user123' exists and contains {name: 'Alice', age: 30}?

Firebase

const docRef = firestore.collection('users').doc('user123');
const docSnap = await docRef.get();
if (docSnap.exists) {
  console.log(docSnap.data());
} else {
  console.log('No such document!');
}

A null

B "No such document!"

C undefined

D {name: 'Alice', age: 30}

💡 Hint

Check what docSnap.data() returns when the document exists.

If the document exists, docSnap.data() returns the document data as an object. Otherwise, it prints 'No such document!'.

🧠 Conceptual

intermediate

Which Firebase method retrieves a single document by its ID?

You want to get a single document from a Firestore collection by its ID. Which method should you use?

A collection('users').doc('user123').get()

B collection('users').get()

C collection('users').where('id', '==', 'user123').get()

D collection('users').add({id: 'user123'})

💡 Hint

Think about how to directly access a document by its ID.

The doc('user123').get() method fetches the single document with that ID. The other options either get all documents or add a new one.

❓ security

advanced

What happens if Firestore security rules deny read access to a document you try to get?

You run this code to get a document:

const docSnap = await firestore.collection('users').doc('user123').get();

But your Firestore security rules deny read access to this document for your user. What will happen?

A The get() call returns null without error.

B The get() call returns an empty object {}.

C The get() call throws a permission error and the document is not returned.

D The get() call returns the document data but logs a warning.

💡 Hint

Consider how Firestore enforces security rules on reads.

If security rules deny read access, Firestore throws a permission error and does not return the document data.

❓ Configuration

advanced

Which code snippet correctly handles the case when a Firestore document does not exist?

You want to get a document and handle the case where it might not exist. Which snippet correctly does this?

A

const docSnap = await firestore.collection('users').doc('user123').get();
if (docSnap === null) {
  console.log('Document missing');
} else {
  console.log(docSnap.data());
}

B

const docSnap = await firestore.collection('users').doc('user123').get();
if (!docSnap.exists) {
  console.log('Document missing');
} else {
  console.log(docSnap.data());
}

C

const docSnap = await firestore.collection('users').doc('user123').get();
if (docSnap.data() === null) {
  console.log('Document missing');
} else {
  console.log(docSnap.data());
}

D

const docSnap = await firestore.collection('users').doc('user123').get();
if (docSnap.data() === undefined) {
  console.log('Document missing');
} else {
  console.log(docSnap.data());
}

💡 Hint

Check the property that indicates document existence.

The exists property on the document snapshot tells if the document exists. Checking data() for null or undefined is not reliable.

❓ Architecture

expert

In a large app, which approach best optimizes reading a single Firestore document frequently?

You have a Firestore document that your app reads very often. Which approach best optimizes performance and cost?

A Use Firestore's offline persistence and cache the document locally to reduce reads.

B Call get() on the document every time you need it to ensure fresh data.

C Use a Cloud Function to read the document and send it to the client on every request.

D Duplicate the document data in Realtime Database for faster reads.

💡 Hint

Think about caching and offline capabilities Firestore offers.

Firestore supports offline persistence and local caching, which reduces network reads and costs while improving performance. Calling get() every time or duplicating data adds overhead or complexity.