Broadcast for custom events in Supabase - Mini Project: Build & Apply

Broadcast for custom events

📖 Scenario: You are building a real-time chat application using Supabase. You want to notify all connected clients when a new message is sent by broadcasting a custom event.

🎯 Goal: Create a Supabase client, configure a channel to broadcast a custom event called new_message, and send a message payload to all subscribers.

📋 What You'll Learn

Create a Supabase client instance with the exact variable name supabase.

Create a channel named chat_channel using supabase.channel().

Broadcast a custom event called new_message with a payload containing { text: 'Hello, world!' }.

Subscribe to the channel to enable broadcasting.

💡 Why This Matters

🌍 Real World

Real-time applications like chat apps, live notifications, or collaborative tools use custom event broadcasting to update all users instantly.

💼 Career

Understanding how to broadcast and subscribe to custom events in Supabase is essential for building scalable real-time cloud applications.

1

Create Supabase client

Create a Supabase client instance called supabase using createClient with the exact URL 'https://xyzcompany.supabase.co' and the API key 'public-anonymous-key'.

Supabase

# Create the Supabase client instance
# Your code here

Hint

Use createClient from @supabase/supabase-js with the given URL and key.

2

Create a channel for broadcasting

Create a channel called chat_channel using supabase.channel('chat_channel').

Supabase

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

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

// Create the channel named 'chat_channel'
# Your code here

Hint

Use the channel method on supabase with the exact name 'chat_channel'.

3

Broadcast the custom event

Use chat_channel.broadcast to send a custom event called new_message with the payload { text: 'Hello, world!' }.

Supabase

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

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

const chat_channel = supabase.channel('chat_channel');

// Broadcast the 'new_message' event with the payload
# Your code here

Hint

Call broadcast on chat_channel with the event name and payload.

4

Subscribe to the channel

Call chat_channel.subscribe() to enable the channel for broadcasting events.

Supabase

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

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

const chat_channel = supabase.channel('chat_channel');

chat_channel.broadcast('new_message', { text: 'Hello, world!' });

// Subscribe to the channel
# Your code here

Hint

Call subscribe() on chat_channel to activate it.

Practice

1. What is the main purpose of using broadcast in Supabase custom events?

easy

A. To let different parts of your app communicate instantly

B. To store data permanently in the database

C. To create user authentication tokens

D. To schedule background jobs automatically

Solution

1. Step 1: Understand the role of broadcast

   Broadcast is used to send messages instantly between parts of an app without delay.

2. Step 2: Compare with other options

   Storing data, authentication, and scheduling jobs are unrelated to broadcast messaging.

3. Final Answer:

   To let different parts of your app communicate instantly -> Option A

4. Quick Check:

   Broadcast = instant app communication [OK]

Hint: Broadcast means instant messaging inside your app [OK]

Common Mistakes:

• Confusing broadcast with database storage
• Thinking broadcast handles authentication
• Assuming broadcast schedules tasks

2. Which of the following is the correct way to listen for a broadcast event on a Supabase channel?

easy

A. channel.subscribe('broadcast', callbackFunction)

B. channel.listen('broadcast', callbackFunction)

C. channel.on('broadcast', callbackFunction)

D. channel.receive('broadcast', callbackFunction)

Solution

1. Step 1: Recall Supabase broadcast listening syntax

   The correct method to listen for broadcast events is on('broadcast', callback).

2. Step 2: Check other options

   Methods like listen, receive, and subscribe are not valid for broadcast events in Supabase.

3. Final Answer:

   channel.on('broadcast', callbackFunction) -> Option C

4. Quick Check:

   Listen with on('broadcast') = channel.on('broadcast', callbackFunction) [OK]

Hint: Use on('broadcast') to listen for events [OK]

Common Mistakes:

• Using listen() instead of on()
• Confusing subscribe() with on()
• Trying receive() which is invalid

3. Given the following code snippet, what will be logged when the broadcast event is sent?

const channel = supabase.channel('chat');
channel.on('broadcast', (payload) => {
  console.log('Received:', payload.message);
});
channel.send({ type: 'broadcast', message: 'Hello!' });

medium

A. Received: Hello!

B. Received: undefined

C. Error: send method not found

D. No output

Solution

1. Step 1: Understand the broadcast sending and listening

   The channel sends a broadcast with message 'Hello!'. The listener logs payload.message.

2. Step 2: Trace the output

   The listener receives the payload and logs 'Received: Hello!'.

3. Final Answer:

   Received: Hello! -> Option A

4. Quick Check:

   Broadcast send triggers on() callback with message [OK]

Hint: send() triggers on('broadcast') with payload [OK]

Common Mistakes:

• Expecting no output because of missing subscription
• Confusing payload structure causing undefined
• Assuming send() method does not exist

4. Identify the error in this Supabase broadcast code snippet:

const channel = supabase.channel('updates');
channel.send({ type: 'broadcast', message: 'Update!' });
channel.on('broadcast', (payload) => {
  console.log(payload.message);
});

medium

A. The send method requires a callback function

B. The payload object must include a 'channel' property

C. The channel name must be 'broadcast' to send broadcast events

D. The listener is set after sending the broadcast, so it misses the message

Solution

1. Step 1: Check the order of send and listen

   The code sends the broadcast before setting the listener, so the listener misses the event.

2. Step 2: Validate other options

   Send does not require a callback; channel name can be any string; payload does not need a 'channel' property.

3. Final Answer:

   The listener is set after sending the broadcast, so it misses the message -> Option D

4. Quick Check:

   Listener must be set before send to catch events [OK]

Hint: Set listener before sending broadcast [OK]

Common Mistakes:

• Sending before listening causes missed events
• Thinking send needs a callback
• Assuming channel name must be 'broadcast'

5. You want to broadcast a custom event to multiple parts of your app grouped under a channel named 'notifications'. Which steps correctly implement this using Supabase's broadcast feature?

hard

A. Use subscribe('notifications') to listen and emit('broadcast') to send events

B. Create a channel 'notifications', set listeners with on('broadcast'), then send events with send({ type: 'broadcast', ... })

C. Create multiple channels for each listener, then send events individually to each channel

D. Broadcast events without creating a channel; listeners listen globally by default

Solution

1. Step 1: Group listeners and senders with a channel

   Create a channel named 'notifications' to group app parts.

2. Step 2: Use on('broadcast') to listen and send() to broadcast

   Listeners use on('broadcast') and senders use send({ type: 'broadcast', ... }) to communicate.

3. Final Answer:

   Create a channel 'notifications', set listeners with on('broadcast'), then send events with send({ type: 'broadcast', ... }) -> Option B

4. Quick Check:

   Channel + on('broadcast') + send() = correct broadcast setup [OK]

Hint: Use one channel with on('broadcast') and send() for group messaging [OK]

Common Mistakes:

• Creating multiple channels unnecessarily
• Using subscribe() or emit() which are invalid
• Assuming broadcast works without channels