Reading data (once and listener) in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

Difference between once() and on() in Firebase Realtime Database

Which option correctly describes the behavior of once() compared to on() when reading data from Firebase Realtime Database?

A <code>once()</code> keeps listening for data changes; <code>on()</code> reads data only once.

B <code>once()</code> reads data only once and then stops listening; <code>on()</code> keeps listening for data changes.

C <code>once()</code> and <code>on()</code> both read data only once and then stop.

D <code>once()</code> and <code>on()</code> both keep listening for data changes indefinitely.

💡 Hint

Think about whether the method keeps watching for updates or just fetches data a single time.

once() fetches data a single time and then stops listening. on() sets up a continuous listener that triggers every time data changes.

❓ Configuration

intermediate

Correct usage of once() to read data once

Which code snippet correctly reads data once from Firebase Realtime Database and logs the value?

Firebase

const dbRef = firebase.database().ref('users/user1');

A dbRef.once('value').then(snapshot => console.log(snapshot.val()));

B dbRef.get('value').then(snapshot => console.log(snapshot.val()));

C dbRef.once('value', snapshot => console.log(snapshot.val()));

D dbRef.on('value', snapshot => console.log(snapshot.val()));

💡 Hint

Check the method that returns a promise and reads data once.

once() returns a promise that resolves with the data snapshot. Passing a callback directly is deprecated. on() sets a listener. get() is a newer method but requires no event type argument.

❓ Architecture

advanced

Choosing between once() and on() for a chat app

You are designing a chat app that shows messages and updates them live. Which approach is best for reading messages?

A Use <code>on()</code> to listen for new messages and update the UI live.

B Use <code>once()</code> repeatedly every few seconds to simulate live updates.

C Use <code>once()</code> to read messages once when the chat opens, then refresh manually.

D Use <code>on()</code> only for the first message, then switch to <code>once()</code>.

💡 Hint

Think about how to keep the chat updated automatically.

on() sets a live listener that updates the UI whenever new messages arrive, which is ideal for chat apps. once() reads data only once and would not update automatically.

❓ security

advanced

Security implications of using on() listeners

What is a key security consideration when using on() listeners in Firebase Realtime Database?

A Listeners only fetch data once, so they have no security impact.

B Listeners automatically encrypt data, so no extra security is needed.

C Listeners can expose data continuously, so rules must restrict read access properly.

D Listeners disable Firebase security rules while active.

💡 Hint

Consider how continuous data access affects security.

on() listeners keep data flowing to clients, so Firebase security rules must carefully control who can read data to prevent leaks.

✅ Best Practice

expert

Proper cleanup of on() listeners to avoid memory leaks

Which code snippet correctly removes a Firebase Realtime Database on() listener to prevent memory leaks?

Firebase

const dbRef = firebase.database().ref('users/user1');
function startListening() {
  dbRef.on('value', callback);
}
function stopListening() {
  // What goes here?
}

A dbRef.removeListener('value', callback);

B dbRef.unsubscribe('value', callback);

C dbRef.stop('value', callback);

D dbRef.off('value', callback);

💡 Hint

Check the Firebase method to remove listeners.

off() is the correct method to remove listeners added by on(). Other methods do not exist or cause errors.