Writing an HTTP Server

HTTP servers are the backbone of the web, allowing you to access websites, download files, and interact with web services. They listen for incoming requests from clients (like web browsers) and send back responses.

When you build your own HTTP server, you have complete control over its behavior and can tailor it to your specific needs. You may be using it for local development, to serve your HTML, CSS, and JS files, or building a REST API - having your own server lets you define endpoints, handle requests and manage data.

Deno's built-in HTTP server Jump to heading#

Deno has a built in HTTP server API that allows you to write HTTP servers. The Deno.serve API supports HTTP/1.1 and HTTP/2.

A "Hello World" server Jump to heading#

The Deno.serve function takes a handler function that will be called for each incoming request, and is expected to return a response (or a promise resolving to a response).

Here is an example of a server that returns a "Hello, World!" response for each request:

Deno.serve((_req) => {
  return new Response("Hello, World!");
});

The handler can also return a Promise<Response>, which means it can be an async function.

To run this server, you can use the deno run command:

deno run --allow-net server.ts

Listening on a specific port Jump to heading#

By default Deno.serve will listen on port 8000, but this can be changed by passing in a port number in options bag as the first or second argument:

// To listen on port 4242.
Deno.serve({ port: 4242 }, handler);

// To listen on port 4242 and bind to 0.0.0.0.
Deno.serve({ port: 4242, hostname: "0.0.0.0" }, handler);

Inspecting the incoming request Jump to heading#

Most servers will not answer with the same response for every request. Instead they will change their answer depending on various aspects of the request: the HTTP method, the headers, the path, or the body contents.

The request is passed in as the first argument to the handler function. Here is an example showing how to extract various parts of the request:

Deno.serve(async (req) => {
  console.log("Method:", req.method);

  const url = new URL(req.url);
  console.log("Path:", url.pathname);
  console.log("Query parameters:", url.searchParams);

  console.log("Headers:", req.headers);

  if (req.body) {
    const body = await req.text();
    console.log("Body:", body);
  }

  return new Response("Hello, World!");
});

Responding with real data Jump to heading#

Most servers do not respond with "Hello, World!" to every request. Instead they might respond with different headers, status codes, and body contents (even body streams).

Here is an example of returning a response with a 404 status code, a JSON body, and a custom header:

Deno.serve((req) => {
  const body = JSON.stringify({ message: "NOT FOUND" });
  return new Response(body, {
    status: 404,
    headers: {
      "content-type": "application/json; charset=utf-8",
    },
  });
});

Responding with a stream Jump to heading#

Response bodies can also be streams. Here is an example of a response that returns a stream of "Hello, World!" repeated every second:

Deno.serve((req) => {
  let timer: number;
  const body = new ReadableStream({
    async start(controller) {
      timer = setInterval(() => {
        controller.enqueue("Hello, World!\n");
      }, 1000);
    },
    cancel() {
      clearInterval(timer);
    },
  });
  return new Response(body.pipeThrough(new TextEncoderStream()), {
    headers: {
      "content-type": "text/plain; charset=utf-8",
    },
  });
});

Be aware that the response body stream is "cancelled" when the client hangs up the connection. Make sure to handle this case. This can surface itself as an error in a write() call on a WritableStream object that is attached to the response body ReadableStream object (for example through a TransformStream).

HTTPS support Jump to heading#

To use HTTPS, pass two extra arguments in the options: cert and key. These are contents of the certificate and key files, respectively.

Deno.serve({
  port: 443,
  cert: Deno.readTextFileSync("./cert.pem"),
  key: Deno.readTextFileSync("./key.pem"),
}, handler);

HTTP/2 support Jump to heading#

HTTP/2 support is "automatic" when using the HTTP server APIs with Deno. You just need to create your server, and it will handle HTTP/1 or HTTP/2 requests seamlessly.

HTTP/2 is also supported over cleartext with prior knowledge.

Automatic body compression Jump to heading#

The HTTP server has built in automatic compression of response bodies. When a response is sent to a client, Deno determines if the response body can be safely compressed. This compression happens within the internals of Deno, so it is fast and efficient.

Currently Deno supports gzip and brotli compression. A body is automatically compressed if the following conditions are true:

• The request has an Accept-Encoding header which indicates the requester supports br for Brotli or gzip. Deno will respect the preference of the quality value in the header.
• The response includes a Content-Type which is considered compressible. (The list is derived from jshttp/mime-db with the actual list in the code.)
• The response body is greater than 64 bytes.

When the response body is compressed, Deno will set the Content-Encoding header to reflect the encoding, as well as ensure the Vary header is adjusted or added to indicate which request headers affected the response.

In addition to the logic above, there are a few reasons why a response won’t be compressed automatically:

• The response contains a Content-Encoding header. This indicates your server has done some form of encoding already.
• The response contains a Content-Range header. This indicates that your server is responding to a range request, where the bytes and ranges are negotiated outside of the control of the internals to Deno.
• The response has a Cache-Control header which contains a no-transform value. This indicates that your server doesn’t want Deno or any downstream proxies to modify the response.

Serving WebSockets Jump to heading#

Deno can upgrade incoming HTTP requests to a WebSocket. This allows you to handle WebSocket endpoints on your HTTP servers.

To upgrade an incoming Request to a WebSocket you use the Deno.upgradeWebSocket function. This returns an object consisting of a Response and a web standard WebSocket object. The returned response should be used to respond to the incoming request.

Because the WebSocket protocol is symmetrical, the WebSocket object is identical to the one that can be used for client side communication. Documentation for it can be found on MDN.

Deno.serve((req) => {
  if (req.headers.get("upgrade") != "websocket") {
    return new Response(null, { status: 501 });
  }

  const { socket, response } = Deno.upgradeWebSocket(req);
  socket.addEventListener("open", () => {
    console.log("a client connected!");
  });

  socket.addEventListener("message", (event) => {
    if (event.data === "ping") {
      socket.send("pong");
    }
  });

  return response;
});

The connection the WebSocket was created on can not be used for HTTP traffic after a WebSocket upgrade has been performed.

Default fetch export Jump to heading#

Another way to create an HTTP server in Deno is by exporting a default fetch function. The fetch API initiates an HTTP request to retrieve data from across a network and is built into the Deno runtime.

export default {
  fetch(request) {
    const userAgent = request.headers.get("user-agent") || "Unknown";
    return new Response(`User Agent: ${userAgent}`);
  },
} satisfies Deno.ServeDefaultExport;

You can run this file with the deno serve command:

deno serve server.ts

The server will start and display a message in the console. Open your browser and navigate to http://localhost:8000/ to see the user-agent information.

Building on these examples Jump to heading#

You will likely want to expand on these examples to create more complex servers. Deno recommends using Oak for building web servers. Oak is a middleware framework for Deno's HTTP server, designed to be expressive and easy to use. It provides a simple way to create web servers with middleware support. Check out the Oak documentation for examples of how to define routes.