WebSockets

Bun.serve() supports server-side WebSockets, with on-the-fly compression, TLS support, and a Bun-native publish-subscribe API.

⚡️ 7x more throughputBun’s WebSockets are fast. For a simple chatroom on Linux x64, Bun can handle 7x more requests per second than Node.js + "ws".

Messages sent per second	Runtime	Clients
~700,000	(`Bun.serve`) Bun v0.2.1 (x64)	16
~100,000	(`ws`) Node v18.10.0 (x64)	16

Internally Bun’s WebSocket implementation is built on uWebSockets.

Start a WebSocket server

The following server, built with Bun.serve, upgrades every incoming request to a WebSocket connection in the fetch handler. The socket handlers are declared in the websocket parameter.

server.ts

Bun.serve({
  fetch(req, server) {
    // upgrade the request to a WebSocket
    if (server.upgrade(req)) {
      return; // do not return a Response
    }
    return new Response("Upgrade failed", { status: 500 });
  },
  websocket: {}, // handlers
});

Bun supports these WebSocket event handlers:

server.ts

Bun.serve({
  fetch(req, server) {}, // upgrade logic
  websocket: {
    message(ws, message) {}, // a message is received
    open(ws) {}, // a socket is opened
    close(ws, code, message) {}, // a socket is closed
    drain(ws) {}, // the socket is ready to receive more data
  },
});

An API designed for speed

In Bun, handlers are declared once per server, instead of per socket.You pass a single WebSocketHandler object to Bun.serve() with methods for open, message, close, drain, and error. This is different from the client-side WebSocket class, which extends EventTarget (onmessage, onopen, onclose).Clients tend to have few socket connections open, so an event-based API makes sense there.But servers tend to have many socket connections open, which means:

Time spent adding/removing event listeners for each connection adds up
Extra memory spent on storing references to callback functions for each connection
Usually, people create new functions for each connection, which also means more memory

Reusing one handler object across every connection avoids both costs.

The first argument to each handler is the ServerWebSocket instance handling the event. The ServerWebSocket class is a fast, Bun-native implementation of WebSocket with some additional features.

server.ts

Bun.serve({
  fetch(req, server) {}, // upgrade logic
  websocket: {
    message(ws, message) {
      ws.send(message); // echo back the message
    },
  },
});

Sending messages

Each ServerWebSocket instance has a .send() method for sending messages to the client. It supports a range of input types.

server.ts

Bun.serve({
  fetch(req, server) {}, // upgrade logic
  websocket: {
    message(ws, message) {
      ws.send("Hello world"); // string
      ws.send(response.arrayBuffer()); // ArrayBuffer
      ws.send(new Uint8Array([1, 2, 3])); // TypedArray | DataView
    },
  },
});

Headers

Once the upgrade succeeds, Bun sends a 101 Switching Protocols response per the spec. To attach additional headers to this Response, pass them to server.upgrade().

server.ts

Bun.serve({
  fetch(req, server) {
    const sessionId = await generateSessionId();
    server.upgrade(req, {
      headers: { 
        "Set-Cookie": `SessionId=${sessionId}`, 
      }, 
    });
  },
  websocket: {}, // handlers
});

Contextual data

Attach contextual data to a new WebSocket in the .upgrade() call. It is available on the ws.data property inside the WebSocket handlers. To strongly type ws.data, add a data property to the websocket handler object. This types ws.data across all lifecycle hooks.

server.ts

type WebSocketData = {
  createdAt: number;
  channelId: string;
  authToken: string;
};

Bun.serve({
  fetch(req, server) {
    const cookies = new Bun.CookieMap(req.headers.get("cookie")!);

    server.upgrade(req, {
      // this object must conform to WebSocketData
      data: {
        createdAt: Date.now(),
        channelId: new URL(req.url).searchParams.get("channelId"),
        authToken: cookies.get("X-Token"),
      },
    });

    return undefined;
  },
  websocket: {
    // TypeScript: specify the type of ws.data like this
    data: {} as WebSocketData,
    // handler called when a message is received
    async message(ws, message) {
      // ws.data is now properly typed as WebSocketData
      const user = getUserFromToken(ws.data.authToken);

      await saveMessageToDatabase({
        channel: ws.data.channelId,
        message: String(message),
        userId: user.id,
      });
    },
  },
});

Previously, you could specify the type of ws.data with a type parameter on Bun.serve, like Bun.serve<MyData>({...}). This pattern was removed due to a limitation in TypeScript in favor of the data property.

To connect to this server from the browser, create a new WebSocket.

browser.js

const socket = new WebSocket("ws://localhost:3000/chat");

socket.addEventListener("message", event => {
  console.log(event.data);
});

Identifying usersCookies set on the page are sent with the WebSocket upgrade request and available on req.headers in the fetch handler. Parse them to identify the connecting user and set data accordingly.

Pub/Sub

Bun’s ServerWebSocket includes a native publish-subscribe API for topic-based broadcasting. Individual sockets can .subscribe() to a topic (specified with a string identifier) and .publish() messages to all other subscribers to that topic (excluding itself). This topic-based broadcast API is similar to MQTT and Redis Pub/Sub.

server.ts

const server = Bun.serve({
  fetch(req, server) {
    const url = new URL(req.url);
    if (url.pathname === "/chat") {
      console.log(`upgrade!`);
      const username = getUsernameFromReq(req);
      const success = server.upgrade(req, { data: { username } });
      return success ? undefined : new Response("WebSocket upgrade error", { status: 400 });
    }

    return new Response("Hello world");
  },
  websocket: {
    // TypeScript: specify the type of ws.data like this
    data: {} as { username: string },
    open(ws) {
      const msg = `${ws.data.username} has entered the chat`;
      ws.subscribe("the-group-chat");
      server.publish("the-group-chat", msg);
    },
    message(ws, message) {
      // this is a group chat
      // so the server re-broadcasts incoming message to everyone
      server.publish("the-group-chat", `${ws.data.username}: ${message}`);

      // inspect current subscriptions
      console.log(ws.subscriptions); // ["the-group-chat"]
    },
    close(ws) {
      const msg = `${ws.data.username} has left the chat`;
      ws.unsubscribe("the-group-chat");
      server.publish("the-group-chat", msg);
    },
  },
});

console.log(`Listening on ${server.hostname}:${server.port}`);

Calling .publish(data) sends the message to all subscribers of a topic except the socket that called .publish(). To send a message to all subscribers of a topic, use the .publish() method on the Server instance.

const server = Bun.serve({
  websocket: {
    // ...
  },
});

// listen for some external event
server.publish("the-group-chat", "Hello world");

Compression

Enable per-message compression with the perMessageDeflate parameter.

server.ts

Bun.serve({
  websocket: {
    perMessageDeflate: true, 
  },
});

To compress an individual message, pass a boolean as the second argument to .send().

ws.send("Hello world", true);

For fine-grained control over compression characteristics, refer to the Reference.

Backpressure

The .send(message) method of ServerWebSocket returns a number indicating the result of the operation.

-1 — The message was enqueued but there is backpressure
0 — The message was dropped due to a connection issue
1+ — The number of bytes sent

Timeouts and limits

By default, Bun closes a WebSocket connection that has been idle for 120 seconds. Configure this with the idleTimeout parameter.

Bun.serve({
  fetch(req, server) {}, // upgrade logic
  websocket: {
    idleTimeout: 60, // 60 seconds
  },
});

Bun also closes a WebSocket connection if it receives a message larger than 16 MB. Configure this with the maxPayloadLength parameter.

Bun.serve({
  fetch(req, server) {}, // upgrade logic
  websocket: {
    maxPayloadLength: 1024 * 1024, // 1 MB
  },
});

Connect to a `Websocket` server

Bun implements the WebSocket class. To create a WebSocket client that connects to a ws:// or wss:// server, create an instance of WebSocket, as you would in the browser.

const socket = new WebSocket("ws://localhost:3000");

// With subprotocol negotiation
const socket2 = new WebSocket("ws://localhost:3000", ["soap", "wamp"]);

In browsers, cookies set on the page are sent with the WebSocket upgrade request. This is a standard feature of the WebSocket API. In Bun, you can also set custom headers directly in the constructor. This is a Bun-specific extension of the WebSocket standard. It does not work in browsers.

const socket = new WebSocket("ws://localhost:3000", {
  headers: {
    /* custom headers */
  }, 
});

To add event listeners to the socket:

// message is received
socket.addEventListener("message", event => {});

// socket opened
socket.addEventListener("open", event => {});

// socket closed
socket.addEventListener("close", event => {});

// error handler
socket.addEventListener("error", event => {});

Reference

See Typescript Definitions

namespace Bun {
  export function serve(params: {
    fetch: (req: Request, server: Server) => Response | Promise<Response>;
    websocket?: {
      message: (ws: ServerWebSocket, message: string | ArrayBuffer | Uint8Array) => void;
      open?: (ws: ServerWebSocket) => void;
      close?: (ws: ServerWebSocket, code: number, reason: string) => void;
      error?: (ws: ServerWebSocket, error: Error) => void;
      drain?: (ws: ServerWebSocket) => void;

      maxPayloadLength?: number; // default: 16 * 1024 * 1024 = 16 MB
      idleTimeout?: number; // default: 120 (seconds)
      backpressureLimit?: number; // default: 16 * 1024 * 1024 = 16 MB
      closeOnBackpressureLimit?: boolean; // default: false
      sendPings?: boolean; // default: true
      publishToSelf?: boolean; // default: false

      perMessageDeflate?:
        | boolean
        | {
            compress?: boolean | Compressor;
            decompress?: boolean | Compressor;
          };
    };
  }): Server;
}

type Compressor =
  | `"disable"`
  | `"shared"`
  | `"dedicated"`
  | `"3KB"`
  | `"4KB"`
  | `"8KB"`
  | `"16KB"`
  | `"32KB"`
  | `"64KB"`
  | `"128KB"`
  | `"256KB"`;

interface Server {
  pendingWebSockets: number;
  publish(topic: string, data: string | ArrayBufferView | ArrayBuffer, compress?: boolean): number;
  upgrade(
    req: Request,
    options?: {
      headers?: HeadersInit;
      data?: any;
    },
  ): boolean;
}

interface ServerWebSocket {
  readonly data: any;
  readonly readyState: number;
  readonly remoteAddress: string;
  readonly subscriptions: string[];
  send(message: string | ArrayBuffer | Uint8Array, compress?: boolean): number;
  close(code?: number, reason?: string): void;
  subscribe(topic: string): void;
  unsubscribe(topic: string): void;
  publish(topic: string, message: string | ArrayBuffer | Uint8Array): void;
  isSubscribed(topic: string): boolean;
  cork(cb: (ws: ServerWebSocket) => void): void;
}

Get Started

Core Runtime

File & Module System

HTTP server

Networking

Data & Storage

Concurrency

Process & System

Interop & Tooling

Utilities

Standards & Compatibility

Contributing

WebSockets

Start a WebSocket server

Sending messages

Headers

Contextual data

Pub/Sub

Compression

Backpressure

Timeouts and limits

Connect to a `Websocket` server

Reference

​Start a WebSocket server

​Sending messages

​Headers

​Contextual data

​Pub/Sub

​Compression

​Backpressure

​Timeouts and limits

​Connect to a Websocket server

​Reference

Start a WebSocket server

Sending messages

Headers

Contextual data

Pub/Sub

Compression

Backpressure

Timeouts and limits

Connect to a `Websocket` server

Reference