WebSockets

WebSockets keep a connection open for bidirectional messages. Use the `ws` package on the Node side.

4 min read Level 2/5 #nodejs#websockets#ws

What you'll learn

Start a WebSocket server with `ws`
Send and receive messages
Know when to choose WS vs SSE

A WebSocket is a single TCP connection that stays open and lets both sides send messages at any time. Real-time chat, live dashboards, multiplayer.

The Most Common Library — `ws`

npm install ws

import { WebSocketServer } from "ws";

const wss = new WebSocketServer({ port: 3001 });

wss.on("connection", (ws) => {
  console.log("client connected");

  ws.on("message", (raw) => {
    const msg = JSON.parse(raw.toString());
    console.log("got", msg);
    ws.send(JSON.stringify({ echo: msg }));
  });

  ws.on("close", () => console.log("client gone"));
});

On the browser:

const ws = new WebSocket("ws://localhost:3001");
ws.onopen   = () => ws.send(JSON.stringify({ hello: "server" }));
ws.onmessage = (e) => console.log("got", JSON.parse(e.data));

Broadcasting

Send a message to every connected client:

wss.on("connection", (ws) => {
  ws.on("message", (raw) => {
    for (const client of wss.clients) {
      if (client.readyState === WebSocket.OPEN) {
        client.send(raw.toString());
      }
    }
  });
});

The skeleton of a chat server.

Run WS on the same port as your HTTP server using “upgrade”:

import { createServer } from "node:http";
import { WebSocketServer } from "ws";

const httpServer = createServer((req, res) => res.end("http"));
const wss = new WebSocketServer({ noServer: true });

httpServer.on("upgrade", (req, socket, head) => {
  if (req.url === "/ws") {
    wss.handleUpgrade(req, socket, head, (ws) => {
      wss.emit("connection", ws, req);
    });
  } else {
    socket.destroy();
  }
});

httpServer.listen(3000);

Now port 3000 serves HTTP + WebSocket on /ws. Same TCP socket.

WS vs SSE

Use Case	Pick
Server → client push only (live feed, AI tokens)	SSE — simpler, auto-reconnect, works over HTTP/2
Two-way (chat, multiplayer, collab)	WebSocket
Browsers with strict proxies	SSE (looks like normal HTTP)

Pick the simpler tool. WS is more powerful but adds complexity.

End of Chapter

That wraps low-level HTTP. Next chapter: Express — most of what you’ll actually ship.

Express →