Make sure you are on the 6-building-a-simple-webserver branch to follow along with this section.

One of the most common use-cases for a JavaScript runtime is a web server. Web servers are a common way of exposing APIs and building full stack data-driven applications.

The Deno standard library contains a server module that lets you implement in a web server in just a few lines of code.

First, import the server module from the Deno standard library into the "app.ts" file.

import { serve } from "https://deno.land/std/http/server.ts";

Next, we'll, define HOSTNAME and PORT constants. For the HOSTNAME, we'll use 0.0.0.0, which is the same as saying "localhost" or "127.0.0.1", but is more compatible with things like Docker.

import { serve } from "https://deno.land/std/http/server.ts";
const PORT = 3000;
const HOSTNAME = "0.0.0.0";

The only thing you need from the server library is the serve function. All you need to do is call that function and pass in the hostname and port to get a new server instance.

import { serve } from "https://deno.land/std/http/server.ts";

const PORT = 3000;
const HOSTNAME = "0.0.0.0";

const server = serve({ hostname: HOSTNAME, port: PORT });

The server has been created, but it doesn't do anything because we aren't handling the requests. As awkward as this may seem, we're going to run an infinite for loop and handle any request that comes in by setting the respond property on the request.

Use a for loop to start the server and listen for any incoming request. The body contains a simple response of "Hello World".

import { serve } from "https://deno.land/std/http/server.ts";

const PORT = 3000;
const HOSTNAME = "0.0.0.0";

const server = serve({ hostname: HOSTNAME, port: PORT });

console.log(`Server is now running on: http://${HOSTNAME}:${PORT}`);

for await (const req of server) {
  req.respond({
    body: "Hello World",
  });
}

Run the program with deno run app.ts. It should tell you that permission is denied and that the allow-net flag is required.

Check file:///home/burkeholland/dev/deno-first-look-exercises/app.ts
error: Uncaught PermissionDenied: network access to "0.0.0.0:3000", run again with the --allow-net flag

Run the program again with the --allow-net flag. Be sure to restrict the access to only port 3000.

deno run --allow-net=0.0.0.0:3000 app.ts

The server should now be running on port 3000. When you visit it in the browser, you should see "Hello World".

Returning HTML

Returning text isn't terribly useful, so instead, return some HTML.

for await (const req of server) {
    req.respond({
      body: "<h1>Hello World</h1>",
    });
}

Make sure you restart the server. You can also run with the --watch flag so that the server is restarted every time you make changes.

OK - that's about as simple a web server as we can get. Let's take a look at some common web server type scenarios.