Presence for online status in Supabase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the Supabase client with your project URL.

Supabase

const supabase = createClient('[1]', 'public-anon-key');

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

Ahttps://your-project.supabase.com

Bhttps://api.supabase.io

Chttps://your-project.supabase.co

Dhttps://supabase.com

Attempts:

3 left

💡 Hint

Common Mistakes

Using the generic supabase.com URL instead of your project URL

Using .com instead of .co in the URL

You need to use your specific Supabase project URL to connect the client properly.

2fill in blank

medium

Complete the code to subscribe to presence changes on the 'online_users' channel.

Supabase

const presenceChannel = supabase.channel('online_users').on('presence', { event: '[1]' }, payload => {
  console.log('Presence update:', payload);
});

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

A'presence'

B'join'

C'message'

D'leave'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'presence' event which is not a valid event name

Using 'message' event which is for chat messages

The 'join' event listens for users coming online in the presence channel.

3fill in blank

hard

Fix the error in the code to correctly track the current user's presence state.

Supabase

await presenceChannel.track({ user_id: [1] });

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

Auser.id

BuserId

CuserID

Duserid

Attempts:

3 left

💡 Hint

Common Mistakes

Using camelCase variations like userId or userID which do not exist

Using lowercase 'userid' which is undefined

The correct property to identify the user is 'user.id' from the authenticated user object.

4fill in blank

hard

Fill both blanks to correctly unsubscribe and remove presence tracking.

Supabase

await presenceChannel.[1]();
await presenceChannel.[2]();

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

Auntrack

Bunsubscribe

Cdisconnect

Dstop

Attempts:

3 left

💡 Hint

Common Mistakes

Calling unsubscribe before untrack causing presence to linger

Using disconnect or stop which are not valid methods here

First, untrack removes the user's presence, then unsubscribe stops listening to the channel.

5fill in blank

hard

Fill all three blanks to create a presence state object with user ID, status, and last active timestamp.

Supabase

const presenceState = {
  id: [1],
  status: [2],
  lastActive: [3]
};

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

Auser.id

B'online'

CDate.now()

D'offline'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'offline' status when user is online

Using a static timestamp instead of Date.now()

The presence state includes the user's ID, their current status as 'online', and the current timestamp.

Practice

1. What is the main purpose of using presence in Supabase realtime channels?

easy

A. To show which users are currently online in real-time

B. To store user passwords securely

C. To send emails to users

D. To create database backups automatically

Solution

1. Step 1: Understand presence feature

   Presence in Supabase is designed to track and show who is online at any moment.

2. Step 2: Identify main use case

   It helps applications display real-time online user lists, not for security or backups.

3. Final Answer:

   To show which users are currently online in real-time -> Option A

4. Quick Check:

   Presence = Online user tracking [OK]

Hint: Presence means tracking who is online live [OK]

Common Mistakes:

• Confusing presence with security features
• Thinking presence handles backups
• Assuming presence sends emails

2. Which of the following is the correct way to listen for presence updates in Supabase realtime channels?

easy

A. channel.on('presence', callback)

B. channel.on('update', callback)

C. channel.on('message', callback)

D. channel.on('sync', callback)

Solution

1. Step 1: Recall Supabase presence event names

   Supabase presence uses the 'sync' event to notify when presence data updates.

2. Step 2: Match event to correct listener

   The 'sync' event triggers the callback with the updated online user list.

3. Final Answer:

   channel.on('sync', callback) -> Option D

4. Quick Check:

   Presence updates use 'sync' event [OK]

Hint: Presence updates come from 'sync' event, not 'presence' [OK]

Common Mistakes:

• Using 'presence' event which does not exist
• Confusing 'message' event with presence updates
• Using 'update' event which is unrelated

3. Given this code snippet, what will be logged when a user joins the presence channel?

const channel = supabase.channel('online-users').on('presence', () => {
  console.log(channel.presenceState())
}).subscribe()

medium

A. An object showing all currently online users

B. An error because presenceState() is not a function

C. An empty array always

D. A string message 'User joined'

Solution

1. Step 1: Understand presenceState() method

   presenceState() returns the current presence data, showing online users grouped by user ID.

2. Step 2: Analyze code behavior on 'presence' event

   When a user joins, 'presence' triggers and logs the updated presence state object.

3. Final Answer:

   An object showing all currently online users -> Option A

4. Quick Check:

   presenceState() = online users object [OK]

Hint: presenceState() returns online users object on presence event [OK]

Common Mistakes:

• Thinking presenceState() is undefined
• Expecting a string message instead of object
• Assuming it returns an empty array always

4. You wrote this code to track online users but no updates appear:

const channel = supabase.channel('online-users')
channel.on('presence', () => {
  console.log('Users online:', channel.presenceState())
})

What is the likely problem?

medium

A. The event name 'presence' is incorrect

B. You forgot to call subscribe() on the channel

C. presenceState() does not exist on channel

D. You need to use 'message' event instead

Solution

1. Step 1: Check channel subscription

   Without calling subscribe(), the channel does not connect and receive events.

2. Step 2: Confirm event and method correctness

   'presence' event and presenceState() are correct, so missing subscribe() is the issue.

3. Final Answer:

   You forgot to call subscribe() on the channel -> Option B

4. Quick Check:

   Must call subscribe() to receive events [OK]

Hint: Always call subscribe() after setting up channel listeners [OK]

Common Mistakes:

• Skipping subscribe() call
• Using wrong event name
• Expecting presenceState() without subscription

5. You want to show a live list of online users with their usernames using Supabase presence. Which approach correctly combines presence tracking and user data fetching?

hard

A. Subscribe to 'update' events and update UI without querying user data

B. Query user profiles once on page load, then listen to 'message' events for updates

C. Listen to 'presence' event, get user IDs from presenceState(), then query user profiles from database

D. Use presenceState() to get usernames directly without extra queries

Solution

1. Step 1: Understand presenceState() data

   presenceState() gives user IDs and metadata, but not full usernames.

2. Step 2: Combine presence with database queries

   After getting IDs from presenceState(), query the database to fetch usernames for display.

3. Final Answer:

   Listen to 'presence' event, get user IDs from presenceState(), then query user profiles from database -> Option C

4. Quick Check:

   Presence + DB query = live usernames [OK]

Hint: Use presenceState() IDs then query DB for usernames [OK]

Common Mistakes:

• Expecting presenceState() to contain usernames
• Not querying user profiles after presence update
• Listening to wrong events like 'message' or 'update'