bun.sh/docs/guides/websocket/upgrade.md

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Upgrade an HTTP request to a WebSocket connection

Inside fetch, use the server.upgrade() function to upgrade an incoming Request to a WebSocket connection. Bun automatically returns a 101 Switching Protocols response if the upgrade succeeds.

Refer to the WebSocket docs for more information on building WebSocket servers.

const server = Bun.serve<{ authToken: string }>({
  fetch(req, server) {
    const success = server.upgrade(req);
    if (success) {
      // Bun automatically returns a 101 Switching Protocols
      // if the upgrade succeeds
      return undefined;
    }

    // handle HTTP request normally
    return new Response("Hello world!");
  },
  websocket: {
    // define websocket handlers
  },
});

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