Broadcasting events

Socket.IO makes it easy to send events to all the connected clients.

info

Please note that broadcasting is a server-only feature.

To all connected clients

io.emit("hello", "world");

caution

Clients that are currently disconnected (or in the process of reconnecting) won't receive the event. Storing this event somewhere (in a database, for example) is up to you, depending on your use case.

To all connected clients except the sender

io.on("connection", (socket) => {
  socket.broadcast.emit("hello", "world");
});

note

In the example above, using socket.emit("hello", "world") (without broadcast flag) would send the event to "client A". You can find the list of all the ways to send an event in the cheatsheet.

With acknowledgements

Starting with Socket.IO 4.5.0, you can now broadcast an event to multiple clients and expect an acknowledgement from each one of them:

io.timeout(5000).emit("hello", "world", (err, responses) => {
  if (err) {
    // some clients did not acknowledge the event in the given delay
  } else {
    console.log(responses); // one response per client
  }
});

All broadcasting forms are supported:

• in a room

io.to("room123").timeout(5000).emit("hello", "world", (err, responses) => {
  // ...
});

• from a specific socket

socket.broadcast.timeout(5000).emit("hello", "world", (err, responses) => {
  // ...
});

• in a namespace

io.of("/the-namespace").timeout(5000).emit("hello", "world", (err, responses) => {
  // ...
});

With multiple Socket.IO servers

Broadcasting also works with multiple Socket.IO servers.

You just need to replace the default adapter by the Redis Adapter or another compatible adapter.

In certain cases, you may want to only broadcast to clients that are connected to the current server. You can achieve this with the local flag:

io.local.emit("hello", "world");

In order to target specific clients when broadcasting, please see the documentation about Rooms.