Version: 4.x

Listening to events

There are several ways to handle events that are transmitted between the server and the client.

EventEmitter methods#

On the server-side, the Socket instance extends the Node.js EventEmitter class.

On the client-side, the Socket instance uses the event emitter provided by the component-emitter library, which exposes a subset of the EventEmitter methods.

socket.on(eventName, listener)#

Adds the listener function to the end of the listeners array for the event named eventName.

socket.on("details", (...args) => {  // ...});

socket.once(eventName, listener)#

Adds a one-time listener function for the event named eventName

socket.once("details", (...args) => {  // ...});

socket.off(eventName, listener)#

Removes the specified listener from the listener array for the event named eventName.

const listener = (...args) => {  console.log(args);}
socket.on("details", listener);
// and then later...socket.off("details", listener);

socket.removeAllListeners([eventName])#

Removes all listeners, or those of the specified eventName.

// for a specific eventsocket.removeAllListeners("details");// for all eventssocket.removeAllListeners();

Catch-all listeners#

Since Socket.IO v3, a new API inspired from the EventEmitter2 library allows to declare catch-all listeners.

This feature is available on both the client and the server.

socket.onAny(listener)#

Adds a listener that will be fired when any event is emitted.

socket.onAny((eventName, ...args) => {  // ...});

socket.prependAny(listener)#

Adds a listener that will be fired when any event is emitted. The listener is added to the beginning of the listeners array.

socket.prependAny((eventName, ...args) => {  // ...});

socket.offAny([listener])#

Removes all catch-all listeners, or the given listener.

const listener = (eventName, ...args) => {  console.log(eventName, args);}
socket.onAny(listener);
// and then later...socket.offAny(listener);
// or all listenerssocket.offAny();

Validation#

The validation of the event arguments is out of the scope of the Socket.IO library.

There are many packages in the JS ecosystem which cover this use case, among them:

Example with joi and acknowledgements:

const Joi = require("joi");
const userSchema = Joi.object({  username: Joi.string().max(30).required(),  email: Joi.string().email().required()});
io.on("connection", (socket) => {  socket.on("create user", (payload, callback) => {    if (typeof callback !== "function") {      // not an acknowledgement      return socket.disconnect();    }    const { error, value } = userSchema.validate(payload);    if (error) {      return callback({        status: "KO",        error      });    }    // do something with the value, and then    callback({      status: "OK"    });  });
});

Error handling#

There is currently no built-in error handling in the Socket.IO library, which means you must catch any error that could be thrown in a listener.

io.on("connection", (socket) => {  socket.on("list items", async (callback) => {    try {      const items = await findItems();      callback({        status: "OK",        items      });    } catch (e) {      callback({        status: "NOK"      });    }  });});

On the server-side, using EventEmitter.captureRejections = true (experimental, see here) might be interesting too, depending on your use case.

require("events").captureRejections = true;
io.on("connection", (socket) => {  socket.on("list products", async () => {    const products = await findProducts();    socket.emit("products", products);  });
  socket[Symbol.for('nodejs.rejection')] = (err) => {    socket.emit("error", err);  };});