# Bun Build a publish-subscribe WebSocket server

> Build a publish-subscribe WebSocket server

- Skill: `jarle/bun-build-a-publish-subscribe-websocket-server` (Agent Skill)
- Install (CLI): `npx skillmds@latest add jarle/bun-build-a-publish-subscribe-websocket-server`
- Raw SKILL.md: https://api.skillmd.com/api/skills/jarle/bun-build-a-publish-subscribe-websocket-server/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: jarle (https://skillmd.com/u/jarle)
- Updated: 2026-09-21
- Page: https://skillmd.com/skills/jarle/bun-build-a-publish-subscribe-websocket-server

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# Build a publish-subscribe WebSocket server

Bun's server-side `WebSocket` API provides a native pub-sub API. Sockets can be subscribed to a set of named channels using `socket.subscribe(<name>)`; messages can be published to a channel using `socket.publish(<name>, <message>)`.

This code snippet implements a simple single-channel chat server.

```ts server.ts icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}}
const server = Bun.serve({
  fetch(req, server) {
    const cookies = req.headers.get("cookie");
    const username = getUsernameFromCookies(cookies);
    const success = server.upgrade(req, { data: { username } });
    if (success) return undefined;

    return new Response("Hello world");
  },
  websocket: {
    // TypeScript: specify the type of ws.data like this
    data: {} as { username: string },

    open(ws) {
      const msg = `${ws.data.username} has entered the chat`;
      ws.subscribe("the-group-chat");
      server.publish("the-group-chat", msg);
    },
    message(ws, message) {
      // the server re-broadcasts incoming messages to everyone
      server.publish("the-group-chat", `${ws.data.username}: ${message}`);
    },
    close(ws) {
      const msg = `${ws.data.username} has left the chat`;
      server.publish("the-group-chat", msg);
      ws.unsubscribe("the-group-chat");
    },
  },
});

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