Presence for online status in Supabase

Introduction

Presence helps you know who is online right now. It shows live user activity simply and clearly.

You want to show which friends are online in a chat app.

You need to display active users on a dashboard.

You want to track if a user is currently connected to your service.

You want to update user status in real-time without refreshing the page.

Syntax

Supabase

channel.on('presence_sync', () => {
  const state = channel.presenceState()
  // state shows who is online
})

Use channel.presenceState() to get the list of online users on a channel.

The presence_sync event updates the current online users list.

Examples

Track online users in room 123 and log the current online users.

Supabase

const channel = supabase.channel('room:123')
channel.on('presence_sync', () => {
  console.log(channel.presenceState())
})

Mark user1 as online in the presence list.

Supabase

channel.track({ user_id: 'user1', status: 'online' })

Remove current user from the online presence list when they disconnect.

Supabase

channel.untrack()

Sample Program

This example connects to a Supabase realtime channel called 'room:lobby'. It tracks a user with ID 'user123' as online. It logs the list of online users whenever it updates. After 5 seconds, it removes the user from the online list and logs the updated state.

Supabase

import { createClient } from '@supabase/supabase-js'

const supabaseUrl = 'https://xyzcompany.supabase.co'
const supabaseKey = 'public-anonymous-key'
const supabase = createClient(supabaseUrl, supabaseKey)

async function trackUserPresence() {
  const channel = supabase.channel('room:lobby')

  channel.on('presence_sync', () => {
    console.log('Current online users:', Object.fromEntries(channel.presenceState()))
  })

  await channel.subscribe()

  // Track current user as online
  channel.track({ user_id: 'user123', status: 'online' })

  // Simulate user leaving after 5 seconds
  setTimeout(() => {
    channel.untrack()
    console.log('User left, updated online users:', Object.fromEntries(channel.presenceState()))
  }, 5000)
}

trackUserPresence()

Important Notes

Always subscribe to the channel before tracking presence.

Presence state is an object showing all users currently online.

Use track() to add yourself and untrack() to remove yourself.

Summary

Presence shows who is online in real-time.

Use Supabase realtime channels and presence API to track users.

Listen to presence_sync events to get updated online user lists.

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'