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 event
socket.removeAllListeners("details");
// for all events
socket.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 listeners
socket.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:

• joi
• ajv
• validatorjs

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);
  };
});