Topic-based messaging in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to subscribe a device to a topic.

Firebase

firebase.messaging().subscribeToTopic(token, [1]);

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

AtopicName

Btoken

C'news'

D'user123'

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the token instead of the topic name.

Not using quotes around the topic name.

The subscribeToTopic method requires the topic name as a string. Here, 'news' is the topic.

2fill in blank

medium

Complete the code to send a message to a topic.

Firebase

admin.messaging().send({ topic: [1], notification: { title: 'Hello', body: 'World' } });

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

A'news'

Btoken

C'user123'

D'deviceToken'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a device token instead of a topic name.

Omitting quotes around the topic name.

The topic field must be the topic name string, such as 'news', to send messages to all subscribers.

3fill in blank

hard

Fix the error in the code to unsubscribe from a topic.

Firebase

firebase.messaging().[1](token, 'news');

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

AsubscribeToTopic

BunsubscribeFromTopic

CsendToTopic

DsendMessage

Attempts:

3 left

💡 Hint

Common Mistakes

Using subscribeToTopic instead of unsubscribeFromTopic.

Using sendToTopic which is for sending messages.

To unsubscribe a device from a topic, use unsubscribeFromTopic method.

4fill in blank

hard

Fill both blanks to create a message payload for topic messaging with a data payload.

Firebase

const message = { topic: [1], data: { [2]: 'value' } };

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

A'sports'

B'score'

C'news'

D'title'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a data key that is not a string.

Mixing topic and data keys.

The topic is set to 'sports' and the data key is 'score' with a value.

5fill in blank

hard

Fill all three blanks to send a notification message with a topic and a condition.

Firebase

const message = { [1]: 'weather', notification: { [2]: 'Alert', [3]: 'Rain expected' }, condition: "'weather' in topics || 'news' in topics" };

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

Atopic

Btitle

Cbody

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'data' instead of 'topic' for the topic key.

Mixing up title and body keys.

The message uses topic to specify the topic, and title and body for notification content.

Practice

1. What is the main purpose of topic-based messaging in Firebase Cloud Messaging?

easy

A. To send messages to multiple devices subscribed to a topic

B. To send messages only to a single device

C. To store messages in a database

D. To authenticate users before sending messages

Solution

1. Step 1: Understand topic-based messaging concept

   Topic-based messaging allows grouping devices under a topic to send messages to all subscribers at once.

2. Step 2: Identify the main purpose

   The main purpose is to send messages to multiple devices subscribed to the same topic efficiently.

3. Final Answer:

   To send messages to multiple devices subscribed to a topic -> Option A

4. Quick Check:

   Topic-based messaging = send to many devices [OK]

Hint: Topic messaging targets groups, not single devices [OK]

Common Mistakes:

• Confusing topic messaging with single device messaging
• Thinking it stores messages in database
• Assuming it handles user authentication

2. Which Firebase function is used to subscribe a device to a topic?

easy

A. unsubscribeFromTopic()

B. sendToTopic()

C. getToken()

D. subscribeToTopic()

Solution

1. Step 1: Identify subscription function

   To add a device to a topic, Firebase provides the subscribeToTopic() function.

2. Step 2: Differentiate from other functions

   sendToTopic() sends messages, getToken() gets device token, unsubscribeFromTopic() removes subscription.

3. Final Answer:

   subscribeToTopic() -> Option D

4. Quick Check:

   Subscribe device = subscribeToTopic() [OK]

Hint: Subscribe uses subscribeToTopic(), not sendToTopic() [OK]

Common Mistakes:

• Confusing sendToTopic() with subscription
• Using getToken() to subscribe
• Mixing unsubscribe with subscribe

3. Given this code snippet, what will happen?

const message = {
  topic: 'news',
  notification: { title: 'Hello', body: 'World' }
};
admin.messaging().sendToTopic(message.topic, message.notification);

medium

A. A notification with title 'Hello' and body 'World' is sent to all devices subscribed to 'news'

B. An error occurs because sendToTopic requires a message object, not separate topic and notification

C. The message is sent only to one device

D. The message is stored but not sent

Solution

1. Step 1: Analyze sendToTopic usage

   The sendToTopic method expects a topic string followed by a payload object. Here, message.topic provides the string 'news' and message.notification provides the payload {title: 'Hello', body: 'World'}.

2. Step 2: Determine the outcome

   This correctly sends the notification to all devices subscribed to the 'news' topic.

3. Final Answer:

   A notification with title 'Hello' and body 'World' is sent to all devices subscribed to 'news' -> Option A

4. Quick Check:

   sendToTopic(topic, payload) sends to subscribers [OK]

Hint: sendToTopic(topic_string, payload_object) sends to topic subscribers [OK]

Common Mistakes:

• Confusing sendToTopic(topic, payload) with send(full_message_object)
• Thinking it sends only to one device
• Believing the message is stored but not sent

4. You wrote this code to send a message to a topic:

const message = {
  topic: 'sports',
  notification: { title: 'Game', body: 'Tonight at 8' }
};
admin.messaging().sendToTopic(message);

But it throws an error. What is the likely cause?

medium

A. sendToTopic expects the message object, but you passed only the topic string

B. The topic field should be inside the notification object

C. sendToTopic expects a topic string, not a message object

D. sendToTopic expects the message object, but you passed the whole object correctly

Solution

1. Step 1: Check sendToTopic parameter type

   In Firebase Admin SDK, sendToTopic expects two parameters: topic string and payload object, not a single message object.

2. Step 2: Identify mismatch

   Passing the whole message object as one argument causes an error because the function signature is different.

3. Final Answer:

   sendToTopic expects a topic string, not a message object -> Option C

4. Quick Check:

   sendToTopic(topic, payload) not sendToTopic(message) [OK]

Hint: sendToTopic needs topic string first, then payload [OK]

Common Mistakes:

• Passing message object instead of topic string
• Putting topic inside notification
• Assuming one argument is accepted

5. You want to send a message only to devices subscribed to both 'news' and 'sports' topics. How can you achieve this using Firebase topic messaging?

hard

A. Send two separate messages, one to 'news' and one to 'sports' topics

B. Use condition parameter with "'news' in topics && 'sports' in topics" in sendToTopic

C. Subscribe devices to a combined topic 'news_sports' and send to that topic

D. Send a message to 'news' topic and filter devices manually

Solution

1. Step 1: Understand topic combination options

   Firebase supports sending messages to multiple topics using conditions with logical operators.

2. Step 2: Use condition parameter for AND logic

   Using condition: "'news' in topics && 'sports' in topics" sends message only to devices subscribed to both topics.

3. Step 3: Evaluate other options

   Sending two messages sends to each topic separately, subscribing to combined topic requires extra setup, filtering manually is inefficient.

4. Final Answer:

   Use condition parameter with "'news' in topics && 'sports' in topics" in sendToTopic -> Option B

5. Quick Check:

   Use condition for multiple topic targeting [OK]

Hint: Use condition with && for multiple topic targeting [OK]

Common Mistakes:

• Sending separate messages instead of using condition
• Creating combined topic unnecessarily
• Filtering devices manually after sending