Emitting events

There are several ways to send events between the server and the client.

Basic emit

The Socket.IO API is inspired from the Node.js EventEmitter, which means you can emit events on one side and register listeners on the other:

// server-side
io.on("connection", (socket) => {
  socket.emit("hello", "world");
});

// client-side
socket.on("hello", (arg) => {
  console.log(arg); // world
});

This also works in the other direction:

// server-side
io.on("connection", (socket) => {
  socket.on("hello", (arg) => {
    console.log(arg); // world
  });
});

// client-side
socket.emit("hello", "world");

You can send any number of arguments, and all serializable datastructures are supported, including binary objects like Buffer or TypedArray.

// server-side
io.on("connection", (socket) => {
  socket.emit("hello", 1, "2", { 3: '4', 5: Buffer.from([6]) });
});

// client-side
socket.on("hello", (arg1, arg2, arg3) => {
  console.log(arg1); // 1
  console.log(arg2); // "2"
  console.log(arg3); // { 3: '4', 5: ArrayBuffer (1) [ 6 ] }
});

There is no need to run JSON.stringify() on objects as it will be done for you.

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

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

Note: Map and Set are not serializable and must be manually serialized:

const serializedMap = [...myMap.entries()];
const serializedSet = [...mySet.keys()];

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.

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

// server-side
io.on("connection", (socket) => {
  socket.on("update item", (arg1, arg2, callback) => {
    console.log(arg1); // 1
    console.log(arg2); // { name: "updated" }
    callback({
      status: "ok"
    });
  });
});

// client-side
socket.emit("update item", "1", { name: "updated" }, (response) => {
  console.log(response.status); // ok
});

Timeout are not supported by default, but it is quite straightforward to implement:

const withTimeout = (onSuccess, onTimeout, timeout) => {
  let called = false;

  const timer = setTimeout(() => {
    if (called) return;
    called = true;
    onTimeout();
  }, timeout);

  return (...args) => {
    if (called) return;
    called = true;
    clearTimeout(timer);
    onSuccess.apply(this, args);
  }
}

socket.emit("hello", 1, 2, withTimeout(() => {
  console.log("success!");
}, () => {
  console.log("timeout!");
}, 1000));

Volatile events

Volatile events are events that will not be sent if the underlying connection is not ready (a bit like UDP, in terms of reliability).

This can be interesting for example if you need to send the position of the characters in an online game (as only the latest values are useful).

socket.volatile.emit("hello", "might or might not be received");

Another use case is to discard events when the client is not connected (by default, the events are buffered until reconnection).

Example:

// server-side
io.on("connection", (socket) => {
  console.log("connect");

  socket.on("ping", (count) => {
    console.log(count);
  });
});

// client-side
let count = 0;
setInterval(() => {
  socket.volatile.emit("ping", ++count);
}, 1000);

If you restart the server, you will see in the console:

connect
1
2
3
4
# the server is restarted, the client automatically reconnects
connect
9
10
11

Without the volatile flag, you would see:

connect
1
2
3
4
# the server is restarted, the client automatically reconnects and sends its buffered events
connect
5
6
7
8
9
10
11