WebSocket Server in Bun

From the Bun cheat sheet · WebSockets · verified Jul 2026

WebSocket Server

Add WebSocket support to Bun.serve with handlers and pub/sub

typescript

Bun.serve({
  fetch(req, server) {
    if (server.upgrade(req)) return; // Upgrade to WS
    return new Response("Not a WS request", { status: 400 });
  },
  websocket: {
    open(ws) { ws.send("Welcome!"); },
    message(ws, msg) { ws.send(`Echo: ${msg}`); },
    close(ws) { console.log("Disconnected"); },
  },
});

💡 Bun WebSockets handle 4x more messages per second than Node.js ws package

⚡ Built-in pub/sub with ws.subscribe/publish — no Redis or external broker needed

📌 Pass per-socket data in server.upgrade() — access it later via ws.data

🟢 WebSocket handlers are defined alongside HTTP routes in the same Bun.serve() call

websocketrealtimepubsub

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