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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.
/icons/typescript.svgserver.ts
const server = Bun.serve<{username: string}>({
	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: {
		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}`);
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