Server - Bun

Basic Setup

https://mintcdn.com/bun-1dd33a4e/JUhaF6Mf68z_zHyy/icons/typescript.svg?fit=max&auto=format&n=JUhaF6Mf68z_zHyy&q=85&s=7ac549adaea8d5487d8fbd58cc3ea35b

index.ts

const server = Bun.serve({
  // `routes` requires Bun v1.2.3+
  routes: {
    // Static routes
    "/api/status": new Response("OK"),

    // Dynamic routes
    "/users/:id": req => {
      return new Response(`Hello User ${req.params.id}!`);
    },

    // Per-HTTP method handlers
    "/api/posts": {
      GET: () => new Response("List posts"),
      POST: async req => {
        const body = await req.json();
        return Response.json({ created: true, ...body });
      },
    },

    // Wildcard route for all routes that start with "/api/" and aren't otherwise matched
    "/api/*": Response.json({ message: "Not found" }, { status: 404 }),

    // Redirect from /blog/hello to /blog/hello/world
    "/blog/hello": Response.redirect("/blog/hello/world"),

    // Serve a file by lazily loading it into memory
    "/favicon.ico": Bun.file("./favicon.ico"),
  },

  // (optional) fallback for unmatched routes:
  // Required if Bun's version < 1.2.3
  fetch(req) {
    return new Response("Not Found", { status: 404 });
  },
});

console.log(`Server running at ${server.url}`);

HTML imports

Import HTML files directly into your server code to build full-stack applications with both server-side and client-side code. HTML imports work in two modes: Development (bun --hot): Bun bundles assets on demand at runtime and enables hot module replacement (HMR): when you change your frontend code, the browser updates without a full page reload. Production (bun build): When you build with bun build --target=bun, the import index from "./index.html" statement resolves to a pre-built manifest object containing all bundled client assets. Bun.serve serves the assets from this manifest with no bundling at runtime.

import myReactSinglePageApp from "./index.html";

Bun.serve({
  routes: {
    "/": myReactSinglePageApp,
  },
});

HTML imports don’t just serve HTML: they run Bun’s bundler, JavaScript transpiler, and CSS parser, so you can build frontends with React, TypeScript, and Tailwind CSS. For a complete guide to building full-stack applications with HTML imports, see fullstack dev server.

Configuration

Changing the `port` and `hostname`

To configure which port and hostname the server listens on, set port and hostname in the options object.

Bun.serve({
  port: 8080, // defaults to $BUN_PORT, $PORT, $NODE_PORT otherwise 3000
  hostname: "mydomain.com", // defaults to "0.0.0.0"
  fetch(req) {
    return new Response("404!");
  },
});

To randomly select an available port, set port to 0.

const server = Bun.serve({
  port: 0, // random port
  fetch(req) {
    return new Response("404!");
  },
});

// server.port is the randomly selected port
console.log(server.port);

Read the chosen port from the server’s port or url property.

console.log(server.port); // 3000
console.log(server.url); // http://localhost:3000

Configuring a default port

Several flags and environment variables set the default port, which Bun uses when the port option is not set.

--port CLI flag

bun --port=4002 server.ts

BUN_PORT environment variable

BUN_PORT=4002 bun server.ts

PORT environment variable

terminal

PORT=4002 bun server.ts

NODE_PORT environment variable

terminal

NODE_PORT=4002 bun server.ts

Unix domain sockets

To listen on a unix domain socket, pass the unix option with the path to the socket.

Bun.serve({
  unix: "/tmp/my-socket.sock", // path to socket
  fetch(req) {
    return new Response(`404!`);
  },
});

Abstract namespace sockets

On Linux, Bun also supports abstract namespace sockets: prefix the unix path with a null byte.

Bun.serve({
  unix: "\0my-abstract-socket", // abstract namespace socket
  fetch(req) {
    return new Response(`404!`);
  },
});

Unlike unix domain sockets, abstract namespace sockets are not bound to the filesystem and are automatically removed when the last reference to the socket is closed.

HTTP/3 (QUIC)

HTTP/3 support in Bun.serve is experimental and may change in future releases.

Bun.serve can also listen for HTTP/3 over QUIC. Set http3: true together with tls; HTTP/3 requires TLS.

Bun.serve({
  tls: {
    key: Bun.file("./key.pem"),
    cert: Bun.file("./cert.pem"),
  },
  http3: true, 
  fetch(req) {
    return new Response("Hello over HTTP/3!");
  },
});

When http3 is enabled, the server listens on the same port over both TCP (HTTP/1.1) and UDP (HTTP/3). HTTP/1.1 responses include an Alt-Svc header advertising the HTTP/3 endpoint so capable clients can upgrade automatically. To serve HTTP/3 only — no TCP listener at all — set http1: false:

Bun.serve({
  tls: {
    key: Bun.file("./key.pem"),
    cert: Bun.file("./cert.pem"),
  },
  http3: true,
  http1: false, 
  fetch(req) {
    return new Response("HTTP/3 only");
  },
});

http3 is not supported with unix domain sockets — QUIC requires a UDP port. http1: false requires http3: true.

idleTimeout

By default, Bun.serve closes connections after 10 seconds of inactivity. A connection is idle when no data is being sent or received, including in-flight requests where your handler is still running but hasn’t written any bytes to the response yet. Browsers and fetch() clients see this as a connection reset. To configure this, set the idleTimeout field (in seconds). The maximum value is 255, and 0 disables the timeout entirely.

Bun.serve({
  // 30 seconds (default is 10)
  idleTimeout: 30,

  fetch(req) {
    return new Response("Bun!");
  },
});

Streaming & Server-Sent Events — The idle timer applies while a response is being streamed. If your stream goes quiet for longer than idleTimeout, Bun closes the connection mid-response. For long-lived streams, disable the timeout for that request with server.timeout(req, 0).

export default syntax

Instead of passing the server options into Bun.serve, you can export default them.

server.ts

import type { Serve } from "bun";

export default {
  fetch(req) {
    return new Response("Bun!");
  },
} satisfies Serve.Options<undefined>;

The type parameter <undefined> is the WebSocket data type. If you add a websocket handler that attaches custom data with server.upgrade(req, { data: ... }), replace undefined with your data type. You can run this file as-is: when Bun sees a file with a default export containing a fetch handler, it passes it into Bun.serve.

Hot Route Reloading

Update routes without server restarts using server.reload():

const server = Bun.serve({
  routes: {
    "/api/version": () => Response.json({ version: "1.0.0" }),
  },
});

// Deploy new routes without downtime
server.reload({
  routes: {
    "/api/version": () => Response.json({ version: "2.0.0" }),
  },
});

Server Lifecycle Methods

`server.stop()`

To stop the server from accepting new connections:

const server = Bun.serve({
  fetch(req) {
    return new Response("Hello!");
  },
});

// Gracefully stop the server (waits for in-flight requests)
await server.stop();

// Force stop and close all active connections
await server.stop(true);

By default, stop() allows in-flight requests and WebSocket connections to complete. Pass true to immediately terminate all connections.

`server.ref()` and `server.unref()`

Control whether the server keeps the Bun process alive:

// Don't keep process alive if server is the only thing running
server.unref();

// Restore default behavior - keep process alive
server.ref();

`server.reload()`

Update the server’s handlers without restarting:

const server = Bun.serve({
  routes: {
    "/api/version": Response.json({ version: "v1" }),
  },
  fetch(req) {
    return new Response("v1");
  },
});

// Update to new handler
server.reload({
  routes: {
    "/api/version": Response.json({ version: "v2" }),
  },
  fetch(req) {
    return new Response("v2");
  },
});

Use this for development and hot reloading. Only fetch, error, and routes can be updated.

Per-Request Controls

`server.timeout(Request, seconds)`

Override the idle timeout for an individual request. Pass 0 to disable the timeout entirely for that request.

const server = Bun.serve({
  async fetch(req, server) {
    // Give this request up to 60 seconds of inactivity instead of the default 10
    server.timeout(req, 60);

    // If they take longer than 60 seconds to send the body, the request will be aborted
    await req.text();

    return new Response("Done!");
  },
});

Use server.timeout(req, 0) to keep a long-lived streaming response (like Server-Sent Events) alive without raising the global idleTimeout for every request:

Bun.serve({
  routes: {
    "/events": (req, server) => {
      // Disable the idle timeout for this streaming response.
      // Otherwise the connection will be closed if no bytes
      // are sent for 10 seconds (the default idleTimeout).
      server.timeout(req, 0);

      return new Response(
        async function* () {
          yield "data: hello\n\n";
          // events can arrive sporadically without the connection being killed
        },
        { headers: { "Content-Type": "text/event-stream" } },
      );
    },
  },
});

`server.requestIP(Request)`

Get client IP and port information:

const server = Bun.serve({
  fetch(req, server) {
    const address = server.requestIP(req);
    if (address) {
      return new Response(`Client IP: ${address.address}, Port: ${address.port}`);
    }
    return new Response("Unknown client");
  },
});

Returns null for closed requests or Unix domain sockets.

Server Metrics

`server.pendingRequests` and `server.pendingWebSockets`

Monitor server activity with built-in counters:

const server = Bun.serve({
  fetch(req, server) {
    return new Response(
      `Active requests: ${server.pendingRequests}\n` + `Active WebSockets: ${server.pendingWebSockets}`,
    );
  },
});

`server.subscriberCount(topic)`

Get count of subscribers for a WebSocket topic:

const server = Bun.serve({
  fetch(req, server) {
    const chatUsers = server.subscriberCount("chat");
    return new Response(`${chatUsers} users in chat`);
  },
  websocket: {
    message(ws) {
      ws.subscribe("chat");
    },
  },
});

Benchmarks

The following Bun and Node.js servers respond Bun! to each incoming Request.

Bun

Bun.serve({
  fetch(req: Request) {
    return new Response("Bun!");
  },
  port: 3000,
});

require("http")
  .createServer((req, res) => res.end("Bun!"))
  .listen(8080);

The Bun.serve server can handle roughly 2.5x more requests per second than Node.js on Linux.

Runtime	Requests per second
Node 16	~64,000
Bun	~160,000

Practical example: REST API

Here’s a basic database-backed REST API using Bun’s router with zero dependencies:

import type { Post } from "./types.ts";
import { Database } from "bun:sqlite";

const db = new Database("posts.db");
db.exec(`
  CREATE TABLE IF NOT EXISTS posts (
    id TEXT PRIMARY KEY,
    title TEXT NOT NULL,
    content TEXT NOT NULL,
    created_at TEXT NOT NULL
  )
`);

Bun.serve({
  routes: {
    // List posts
    "/api/posts": {
      GET: () => {
        const posts = db.query("SELECT * FROM posts").all();
        return Response.json(posts);
      },

      // Create post
      POST: async req => {
        const post: Omit<Post, "id" | "created_at"> = await req.json();
        const id = crypto.randomUUID();

        db.query(
          `INSERT INTO posts (id, title, content, created_at)
           VALUES (?, ?, ?, ?)`,
        ).run(id, post.title, post.content, new Date().toISOString());

        return Response.json({ id, ...post }, { status: 201 });
      },
    },

    // Get post by ID
    "/api/posts/:id": req => {
      const post = db.query("SELECT * FROM posts WHERE id = ?").get(req.params.id);

      if (!post) {
        return new Response("Not Found", { status: 404 });
      }

      return Response.json(post);
    },
  },

  error(error) {
    console.error(error);
    return new Response("Internal Server Error", { status: 500 });
  },
});

Reference

See TypeScript Definitions

interface Server extends Disposable {
  /**
   * Stop the server from accepting new connections.
   * @param closeActiveConnections If true, immediately terminates all connections
   * @returns Promise that resolves when the server has stopped
   */
  stop(closeActiveConnections?: boolean): Promise<void>;

  /**
   * Update handlers without restarting the server.
   * Only fetch and error handlers can be updated.
   */
  reload(options: Serve): void;

  /**
   * Make a request to the running server.
   * Useful for testing or internal routing.
   */
  fetch(request: Request | string): Response | Promise<Response>;

  /**
   * Upgrade an HTTP request to a WebSocket connection.
   * @returns true if upgrade successful, false if failed
   */
  upgrade<T = undefined>(
    request: Request,
    options?: {
      headers?: Bun.HeadersInit;
      data?: T;
    },
  ): boolean;

  /**
   * Publish a message to all WebSocket clients subscribed to a topic.
   * @returns Bytes sent, 0 if dropped, -1 if backpressure applied
   */
  publish(
    topic: string,
    data: string | ArrayBufferView | ArrayBuffer | SharedArrayBuffer,
    compress?: boolean,
  ): ServerWebSocketSendStatus;

  /**
   * Get count of WebSocket clients subscribed to a topic.
   */
  subscriberCount(topic: string): number;

  /**
   * Get client IP address and port.
   * @returns null for closed requests or Unix sockets
   */
  requestIP(request: Request): SocketAddress | null;

  /**
   * Set custom idle timeout for a request.
   * @param seconds Timeout in seconds, 0 to disable
   */
  timeout(request: Request, seconds: number): void;

  /**
   * Keep process alive while server is running.
   */
  ref(): void;

  /**
   * Allow process to exit if server is only thing running.
   */
  unref(): void;

  /** Number of in-flight HTTP requests */
  readonly pendingRequests: number;

  /** Number of active WebSocket connections */
  readonly pendingWebSockets: number;

  /** Server URL including protocol, hostname and port */
  readonly url: URL;

  /** Port server is listening on */
  readonly port: number;

  /** Hostname server is bound to */
  readonly hostname: string;

  /** Whether server is in development mode */
  readonly development: boolean;

  /** Server instance identifier */
  readonly id: string;
}

interface WebSocketHandler<T = undefined> {
  /** Maximum WebSocket message size in bytes */
  maxPayloadLength?: number;

  /** Bytes of queued messages before applying backpressure */
  backpressureLimit?: number;

  /** Whether to close connection when backpressure limit hit */
  closeOnBackpressureLimit?: boolean;

  /** Called when backpressure is relieved */
  drain?(ws: ServerWebSocket<T>): void | Promise<void>;

  /** Seconds before idle timeout */
  idleTimeout?: number;

  /** Enable per-message deflate compression */
  perMessageDeflate?:
    | boolean
    | {
        compress?: WebSocketCompressor | boolean;
        decompress?: WebSocketCompressor | boolean;
      };

  /** Send ping frames to keep connection alive */
  sendPings?: boolean;

  /** Whether server receives its own published messages */
  publishToSelf?: boolean;

  /** Called when connection opened */
  open?(ws: ServerWebSocket<T>): void | Promise<void>;

  /** Called when message received */
  message(ws: ServerWebSocket<T>, message: string | Buffer): void | Promise<void>;

  /** Called when connection closed */
  close?(ws: ServerWebSocket<T>, code: number, reason: string): void | Promise<void>;

  /** Called when ping frame received */
  ping?(ws: ServerWebSocket<T>, data: Buffer): void | Promise<void>;

  /** Called when pong frame received */
  pong?(ws: ServerWebSocket<T>, data: Buffer): void | Promise<void>;
}

interface TLSOptions {
  /** Certificate authority chain */
  ca?: string | Buffer | BunFile | Array<string | Buffer | BunFile>;

  /** Server certificate */
  cert?: string | Buffer | BunFile | Array<string | Buffer | BunFile>;

  /** Path to DH parameters file */
  dhParamsFile?: string;

  /** Private key */
  key?: string | Buffer | BunFile | Array<string | Buffer | BunFile>;

  /** Reduce TLS memory usage */
  lowMemoryMode?: boolean;

  /** Private key passphrase */
  passphrase?: string;

  /** OpenSSL options flags */
  secureOptions?: number;

  /** Server name for SNI */
  serverName?: string;
}

​Basic Setup

​HTML imports

​Configuration

​Changing the port and hostname

​Configuring a default port

​Unix domain sockets

​Abstract namespace sockets

​HTTP/3 (QUIC)

​idleTimeout

​export default syntax

​Hot Route Reloading

​Server Lifecycle Methods

​server.stop()

​server.ref() and server.unref()

​server.reload()

​Per-Request Controls

​server.timeout(Request, seconds)

​server.requestIP(Request)

​Server Metrics

​server.pendingRequests and server.pendingWebSockets

​server.subscriberCount(topic)

​Benchmarks

​Practical example: REST API

​Reference

Basic Setup

HTML imports

Configuration

Changing the `port` and `hostname`

Configuring a default port

Unix domain sockets

Abstract namespace sockets

HTTP/3 (QUIC)

idleTimeout

export default syntax

Hot Route Reloading

Server Lifecycle Methods

`server.stop()`

`server.ref()` and `server.unref()`

`server.reload()`

Per-Request Controls

`server.timeout(Request, seconds)`

`server.requestIP(Request)`

Server Metrics

`server.pendingRequests` and `server.pendingWebSockets`

`server.subscriberCount(topic)`

Benchmarks

Practical example: REST API

Reference