---
title: "Deno"
description: "Serve Cap'n Web from Deno.serve, handling both HTTP batch and WebSocket upgrades."
---

> Documentation Index
> Fetch the complete documentation index at: https://255.pr.capnweb.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Deno

Import the package with the `npm:` specifier and use the standard Fetch-API helpers.

```ts
import {
  newHttpBatchRpcResponse,
  newWebSocketRpcSession,
  RpcTarget,
} from 'npm:capnweb';

// This is the server implementation.
class MyApiImpl extends RpcTarget implements MyApi {
  // ... define API, same as above ...
}

Deno.serve(async (req) => {
  const url = new URL(req.url);
  if (url.pathname === '/api') {
if (req.headers.get('upgrade') === 'websocket') {
  const { socket, response } = Deno.upgradeWebSocket(req);
  socket.addEventListener('open', () => {
    newWebSocketRpcSession(socket, new MyApiImpl());
  });
  return response;
} else {
  const response = await newHttpBatchRpcResponse(req, new MyApiImpl());
  // If you are accepting WebSockets, then you might as well accept
  // cross-origin HTTP, since WebSockets always permit cross-origin requests
  // anyway. But see security considerations for further discussion.
  response.headers.set('Access-Control-Allow-Origin', '*');
  return response;
}
  }

  return new Response('Not Found', { status: 404 });
});
```

:::note
Start the RPC session from the `open` event, not immediately after `upgradeWebSocket()`. The socket
is not ready to send until then.
:::

Run it with network permission:

```sh
deno run --allow-net server.ts
```

Source: https://255.pr.capnweb.com/servers/deno/index.mdx