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Overview

Presence allows you to track which users are currently online and synchronize state across clients in real-time. It’s perfect for showing “who’s online” indicators, collaborative features, and multiplayer games.
Presence automatically handles connection and disconnection events, removing users when they leave or lose connection.

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Key Features

  • Automatic cleanup: Users are automatically removed when they disconnect
  • Conflict resolution: Built-in CRDT ensures consistent state across clients
  • Scalable: Efficiently handles thousands of concurrent users
  • Flexible state: Track any JSON-serializable user data

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Basic Usage

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Track Your Presence

Add yourself to a channel’s presence: 
import { createClient } from '@supabase/supabase-js'

const supabase = createClient(
  'https://your-project.supabase.co',
  'your-anon-key'
)

const channel = supabase.channel('online-users', {
  config: {
    presence: {
      key: 'user-123' // Unique identifier for this user
    }
  }
})

channel.subscribe(async (status) => {
  if (status === 'SUBSCRIBED') {
    // Track your presence
    await channel.track({
      user_name: 'Alice',
      online_at: new Date().toISOString()
    })
  }
})

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Get Current Presence State

Retrieve all users currently present: 
channel.on('presence', { event: 'sync' }, () => {
  const state = channel.presenceState()
  console.log('Online users:', state)
})
The presence state is an object where keys are user identifiers: 
{
  'user-123': [
    {
      user_name: 'Alice',
      online_at: '2024-03-04T10:30:00Z'
    }
  ],
  'user-456': [
    {
      user_name: 'Bob',
      online_at: '2024-03-04T10:31:00Z'
    }
  ]
}

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Complete Example

Here’s a real implementation based on the nextjs-auth-presence example in the source: 
import { useSupabaseClient, useUser } from '@supabase/auth-helpers-react'
import { RealtimePresenceState } from '@supabase/supabase-js'
import { useEffect, useState } from 'react'

export default function OnlineUsers() {
  const supabaseClient = useSupabaseClient()
  const user = useUser()
  const [onlineUsers, setOnlineUsers] = useState<RealtimePresenceState>({})

  useEffect(() => {
    const channel = supabaseClient.channel('online-users', {
      config: {
        presence: {
          key: user?.email || 'anonymous'
        }
      }
    })

    // Listen for presence changes
    channel.on('presence', { event: 'sync' }, () => {
      const presenceState = channel.presenceState()
      console.log('Presence state updated:', presenceState)
      setOnlineUsers({ ...presenceState })
    })

    channel.on('presence', { event: 'join' }, ({ newPresences }) => {
      console.log('New users joined:', newPresences)
    })

    channel.on('presence', { event: 'leave' }, ({ leftPresences }) => {
      console.log('Users left:', leftPresences)
    })

    // Subscribe and track presence
    channel.subscribe(async (status) => {
      if (status === 'SUBSCRIBED') {
        const presenceTrackStatus = await channel.track({
          user_name: user?.email || 'anonymous',
          online_at: new Date().toISOString()
        })
        console.log('Presence track status:', presenceTrackStatus)
      }
    })

    // Cleanup
    return () => {
      channel.unsubscribe()
    }
  }, [user])

  return (
    <div>
      <h2>Currently Online Users:</h2>
      <ul>
        {Object.keys(onlineUsers).map((key) => (
          <li key={key}>{key}</li>
        ))}
      </ul>
    </div>
  )
}

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Presence Events

Presence emits three types of events:

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Sync Event

Fired when the presence state is synchronized: 
channel.on('presence', { event: 'sync' }, () => {
  const state = channel.presenceState()
  console.log('Current state:', state)
})

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Join Event

Fired when new users join: 
channel.on('presence', { event: 'join' }, ({ newPresences }) => {
  console.log('Joined:', newPresences)
  // newPresences: Array of user presence data
})

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Leave Event

Fired when users leave or disconnect: 
channel.on('presence', { event: 'leave' }, ({ leftPresences }) => {
  console.log('Left:', leftPresences)
  // leftPresences: Array of user presence data that left
})

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Advanced Patterns

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Update Presence State

Update your presence data without reconnecting: 
const channel = supabase.channel('game-lobby')

await channel.track({
  username: 'Alice',
  status: 'idle'
})

// Later, update status
await channel.track({
  username: 'Alice',
  status: 'playing',
  current_level: 5
})

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Collaborative Editing Example

Track cursor positions and selections in a collaborative editor: 
import { createClientComponentClient } from '@supabase/auth-helpers-nextjs'
import { User, RealtimeChannel } from '@supabase/supabase-js'
import { useState, useEffect } from 'react'

interface CursorPosition {
  email: string
  cursor: { x: number; y: number }
  selection?: { start: number; end: number }
  color: string
}

export default function CollaborativeEditor() {
  const [users, setUsers] = useState<Set<string>>(new Set())
  const [channel, setChannel] = useState<RealtimeChannel | null>(null)
  const supabase = createClientComponentClient()

  useEffect(() => {
    const docChannel = supabase.channel('document-123', {
      config: {
        presence: { key: currentUser.email }
      }
    })

    // Track cursor movements
    docChannel
      .on('presence', { event: 'join' }, ({ newPresences }) => {
        newPresences.forEach(({ email }) => {
          users.add(email)
        })
        setUsers(new Set(users))
      })
      .on('presence', { event: 'leave' }, ({ leftPresences }) => {
        leftPresences.forEach(({ email }) => {
          users.delete(email)
        })
        setUsers(new Set(users))
      })
      .subscribe(async (status) => {
        if (status === 'SUBSCRIBED') {
          await docChannel.track({
            email: currentUser.email,
            cursor: { x: 0, y: 0 },
            color: getRandomColor()
          })
          setChannel(docChannel)
        }
      })

    return () => {
      docChannel.unsubscribe()
    }
  }, [])

  function updateCursor(x: number, y: number) {
    channel?.track({
      email: currentUser.email,
      cursor: { x, y },
      color: userColor
    })
  }

  return (
    <div>
      <div className="online-users">
        {Array.from(users).map(email => (
          <span key={email}>{email}</span>
        ))}
      </div>
      <div
        className="editor"
        onMouseMove={(e) => updateCursor(e.clientX, e.clientY)}
      >
        {/* Editor content */}
      </div>
    </div>
  )
}

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Multiplayer Game Lobby

Track players in a game lobby: 
interface Player {
  id: string
  username: string
  ready: boolean
  team?: 'red' | 'blue'
}

const lobby = supabase.channel('game-lobby-1', {
  config: {
    presence: { key: playerId }
  }
})

// Join lobby
lobby.subscribe(async (status) => {
  if (status === 'SUBSCRIBED') {
    await lobby.track({
      username: playerName,
      ready: false,
      team: null
    })
  }
})

// Mark as ready
function setReady(ready: boolean) {
  lobby.track({
    username: playerName,
    ready,
    team: selectedTeam
  })
}

// Check if all players are ready
lobby.on('presence', { event: 'sync' }, () => {
  const state = lobby.presenceState()
  const players = Object.values(state).flat()
  const allReady = players.every((p: Player) => p.ready)

  if (allReady && players.length >= 2) {
    startGame()
  }
})

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Private Channels with Presence

Combine presence with authorization for private spaces: 
const privateRoom = supabase.channel('private-team-room', {
  config: {
    private: true, // Requires authorization
    presence: { key: userId }
  }
})

privateRoom
  .on('presence', { event: 'sync' }, () => {
    const teamMembers = privateRoom.presenceState()
    console.log('Team members online:', Object.keys(teamMembers))
  })
  .subscribe(async (status, err) => {
    if (status === 'SUBSCRIBED') {
      await privateRoom.track({
        username: currentUser.name,
        role: currentUser.role
      })
    }
    if (status === 'CHANNEL_ERROR') {
      console.error('Not authorized:', err)
    }
  })

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Combining Presence with Broadcast

Use both features together for rich collaborative experiences: 
const channel = supabase.channel('collaboration', {
  config: {
    presence: { key: userId },
    broadcast: { self: true }
  }
})

// Track who's online with presence
channel
  .on('presence', { event: 'sync' }, () => {
    const users = channel.presenceState()
    updateOnlineUsersList(users)
  })
  // Send messages with broadcast
  .on('broadcast', { event: 'message' }, ({ payload }) => {
    displayMessage(payload)
  })
  .subscribe(async (status) => {
    if (status === 'SUBSCRIBED') {
      // Track presence
      await channel.track({
        username: currentUser.name,
        avatar: currentUser.avatar
      })
    }
  })

// Send a message
channel.send({
  type: 'broadcast',
  event: 'message',
  payload: { text: 'Hello!', from: userId }
})

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Performance Considerations

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Throttle Presence Updates

Avoid updating presence too frequently: 
import { throttle } from 'lodash'

// Update at most once per second
const updatePresence = throttle((data) => {
  channel.track(data)
}, 1000)

// Use throttled version
document.addEventListener('mousemove', (e) => {
  updatePresence({
    cursor: { x: e.clientX, y: e.clientY }
  })
})

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Limit Presence State Size

Keep presence data small for better performance: 
// Good: Small, essential data
await channel.track({
  username: 'alice',
  status: 'active'
})

// Avoid: Large, unnecessary data
await channel.track({
  username: 'alice',
  profilePicture: 'base64...', // Too large!
  fullHistory: [...] // Unnecessary
})

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Clean Up on Unmount

Always unsubscribe when components unmount: 
useEffect(() => {
  const channel = supabase.channel('presence')
  // ... setup

  return () => {
    channel.unsubscribe() // Important!
  }
}, [])

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Troubleshooting

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User Not Appearing in Presence State

  1. Verify subscription status:
channel.subscribe((status) => {
  console.log('Status:', status)
})
  1. Check if track() was called:
const trackStatus = await channel.track({ username: 'alice' })
console.log('Track status:', trackStatus)
  1. Ensure unique presence keys:
// Each user needs a unique key
const channel = supabase.channel('users', {
  config: {
    presence: { key: user.id } // Must be unique!
  }
})

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Users Not Being Removed

Presence automatically removes users after 60 seconds of inactivity. If a user appears stuck:
  • Check network connectivity
  • Verify the channel is still subscribed
  • Check browser console for errors

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Next Steps

Broadcast

Send ephemeral messages between clients

Postgres Changes

Listen to database changes in real-time