Node.js
emit and on
Learn events with socket.emit and socket.on.
socket.emit and socket.on
socket.emit(event, data) sends an event to the client. socket.on(event, callback) listens to events from the client. These two methods are the cornerstone of Socket.io.
Example: socket.emit("hello", { name: "Ayşe" }); sends the hello event with { name: "Ayşe" } data to the client. socket.on("message", (data) => console.log(data)); listens to message events from the client. emit sends, on listens. Event names are free; client and server must use the same name. Data is sent as the second parameter; it can be an object, array or primitive.
Message exchange
A real chat flow looks like this: the client sends a message with socket.emit("chat", message); the server listens with socket.on("chat", ...). The server broadcasts the message to all clients with io.emit("chat", message).
Example: socket.on("chat", (message) => { io.emit("chat", message); });. Here io.emit sends to all clients; socket.emit sends to one. The difference matters: send private replies to one client, broadcasts to everyone. The event name (chat) must be the same on both sides; otherwise the message is not received.
Exercise Define a chat handler
Use the io object below. Define a chat listener with io.on("connection", (socket) => { socket.on("chat", (mesaj) => { console.log("Got: " + mesaj); }); }). Print "emit/on ready" with console.log.
Show example solution
const socketIo = require("socket.io");
const io = socketIo();
io.on("connection", (socket) => {
socket.on("chat", (mesaj) => { console.log("Got: " + mesaj); });
});
console.log("emit/on ready");
Test yourself
-
Match each method to its role
- socket.emit
- Sends to client
- socket.on
- Listens from client
- event name
- Communication channel
- data
- Payload sent
-
Which one sends a message to a single client?
socket.emit("mesaj", veri)
- io.send(veri)
- socket.broadcast.emit("mesaj", veri)
- socket.emit("mesaj", veri)
- io.emit("mesaj", veri)
Show answer
socket.emit("mesaj", veri) — socket.emit sends to a single client.
-
Complete the socket.on call
socket.on("chat", (____) => { console.log("Message"); });
Show answer
socket.on("chat", (mesaj) => { console.log("Message"); });
Next lesson: Rooms
