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Version: 4.x

Overview of the API

Before we go any further, let's take a quick tour of the API provided by Socket.IO:

Common API

The following methods are available for both the client and the server.

Basic emit

As we have seen in step #4, you can send any data to the other side with socket.emit():

Client

socket.emit('hello', 'world');

Server

io.on('connection', (socket) => {
socket.on('hello', (arg) => {
console.log(arg); // 'world'
});
});

You can send any number of arguments, and all serializable data structures are supported, including binary objects like ArrayBuffer, TypedArray or Buffer (Node.js only):

Client

socket.emit('hello', 1, '2', { 3: '4', 5: Uint8Array.from([6]) });

Server

io.on('connection', (socket) => {
socket.on('hello', (arg1, arg2, arg3) => {
console.log(arg1); // 1
console.log(arg2); // '2'
console.log(arg3); // { 3: '4', 5: <Buffer 06> }
});
});
tip

Calling JSON.stringify() on objects is not needed:

// BAD
socket.emit('hello', JSON.stringify({ name: 'John' }));

// GOOD
socket.emit('hello', { name: 'John' });

Acknowledgements

Events are great, but in some cases you may want a more classic request-response API. In Socket.IO, this feature is named "acknowledgements".

It comes in two flavors:

With a callback function

You can add a callback as the last argument of the emit(), and this callback will be called once the other side has acknowledged the event:

Client

socket.timeout(5000).emit('request', { foo: 'bar' }, 'baz', (err, response) => {
if (err) {
// the server did not acknowledge the event in the given delay
} else {
console.log(response.status); // 'ok'
}
});

Server

io.on('connection', (socket) => {
socket.on('request', (arg1, arg2, callback) => {
console.log(arg1); // { foo: 'bar' }
console.log(arg2); // 'baz'
callback({
status: 'ok'
});
});
});

With a Promise

The emitWithAck() method provides the same functionality, but returns a Promise which will resolve once the other side acknowledges the event:

Client

try {
const response = await socket.timeout(5000).emitWithAck('request', { foo: 'bar' }, 'baz');
console.log(response.status); // 'ok'
} catch (e) {
// the server did not acknowledge the event in the given delay
}

Server

io.on('connection', (socket) => {
socket.on('request', (arg1, arg2, callback) => {
console.log(arg1); // { foo: 'bar' }
console.log(arg2); // 'baz'
callback({
status: 'ok'
});
});
});
caution

Environments that do not support Promises (such as Internet Explorer) will need to add a polyfill or use a compiler like babel in order to use this feature (but this is out of the scope of this tutorial).

Catch-all listeners

A catch-all listeners is a listener that will be called for any incoming event. This is useful for debugging your application:

Sender

socket.emit('hello', 1, '2', { 3: '4', 5: Uint8Array.from([6]) });

Receiver

socket.onAny((eventName, ...args) => {
console.log(eventName); // 'hello'
console.log(args); // [ 1, '2', { 3: '4', 5: ArrayBuffer (1) [ 6 ] } ]
});

Similarly, for outgoing packets:

socket.onAnyOutgoing((eventName, ...args) => {
console.log(eventName); // 'hello'
console.log(args); // [ 1, '2', { 3: '4', 5: ArrayBuffer (1) [ 6 ] } ]
});

Server API

Broadcasting

As we have seen in step #5, you can broadcast an event to all connected clients with io.emit():

io.emit('hello', 'world');
The 'hello' event is sent to all connected clientsThe 'hello' event is sent to all connected clients

Rooms

In Socket.IO jargon, a room is an arbitrary channel that sockets can join and leave. It can be used to broadcast events to a subset of connected clients:

io.on('connection', (socket) => {
// join the room named 'some room'
socket.join('some room');

// broadcast to all connected clients in the room
io.to('some room').emit('hello', 'world');

// broadcast to all connected clients except those in the room
io.except('some room').emit('hello', 'world');

// leave the room
socket.leave('some room');
});
The 'hello' event is sent to all connected clients in the targeted roomThe 'hello' event is sent to all connected clients in the targeted room

That's basically it! For future reference, the whole API can be found here (server) and here (client).