Set per-socket contextual data on a WebSocket - Bun

When building a WebSocket server, it’s typically necessary to store some identifying information or context associated with each connected client. With Bun.serve(), this “contextual data” is set when the connection is initially upgraded by passing a data parameter in the server.upgrade() call.

server.ts

Bun.serve({
  fetch(req, server) {
    const success = server.upgrade(req, {
      data: {
        socketId: Math.random(),
      },
    });
    if (success) return undefined;

    // handle HTTP request normally
    // ...
  },
  websocket: {
    // TypeScript: specify the type of ws.data like this
    data: {} as { socketId: number },

    // define websocket handlers
    async message(ws, message) {
      // the contextual data is available as the `data` property
      // on the WebSocket instance
      console.log(`Received ${message} from ${ws.data.socketId}}`);
    },
  },
});

It’s common to read cookies/headers from the incoming request to identify the connecting client.

server.ts

type WebSocketData = {
  createdAt: number;
  token: string;
  userId: string;
};

Bun.serve({
  async fetch(req, server) {
    // use a library to parse cookies
    const cookies = parseCookies(req.headers.get("Cookie"));
    const token = cookies["X-Token"];
    const user = await getUserFromToken(token);

    const upgraded = server.upgrade(req, {
      data: {
        createdAt: Date.now(),
        token: cookies["X-Token"],
        userId: user.id,
      },
    });

    if (upgraded) return undefined;
  },
  websocket: {
    // TypeScript: specify the type of ws.data like this
    data: {} as WebSocketData,

    async message(ws, message) {
      // save the message to a database
      await saveMessageToDatabase({
        message: String(message),
        userId: ws.data.userId,
      });
    },
  },
});