bun-websocket-server

Bun WebSocket Server

Bun has built-in WebSocket support integrated with Bun.serve().

Quick Start

const server = Bun.serve({
  fetch(req, server) {
    // Upgrade to WebSocket
    if (server.upgrade(req)) {
      return; // Upgraded successfully
    }
    return new Response("Not a WebSocket request", { status: 400 });
  },
  websocket: {
    open(ws) {
      console.log("Client connected");
    },
    message(ws, message) {
      console.log("Received:", message);
      ws.send(`Echo: ${message}`);
    },
    close(ws) {
      console.log("Client disconnected");
    },
  },
});

console.log(`WebSocket server running on ws://localhost:${server.port}`);

WebSocket Handlers

Bun.serve({
  fetch(req, server) {
    server.upgrade(req);
  },
  websocket: {
    // Client connected
    open(ws) {
      console.log("New connection");
    },

    // Message received
    message(ws, message) {
      // message is string | Buffer
      if (typeof message === "string") {
        console.log("Text:", message);
      } else {
        console.log("Binary:", message);
      }
    },

    // Connection closed
    close(ws, code, reason) {
      console.log(`Closed: ${code} - ${reason}`);
    },

    // Drain event (buffer flushed)
    drain(ws) {
      console.log("Buffer drained");
    },

    // Ping received
    ping(ws, data) {
      // Pong sent automatically
    },

    // Pong received
    pong(ws, data) {
      console.log("Pong received");
    },
  },
});

Sending Messages

websocket: {
  message(ws, message) {
    // Send text
    ws.send("Hello");

    // Send JSON
    ws.send(JSON.stringify({ type: "greeting", data: "Hello" }));

    // Send binary
    ws.send(new Uint8Array([1, 2, 3]));
    ws.send(Buffer.from("binary data"));

    // Send with compression
    ws.send("compressed message", true);

    // Check if buffer is full
    const bufferedAmount = ws.send("data");
    if (bufferedAmount > 1024 * 1024) {
      console.log("Buffer getting full");
    }
  },
}