First Steps

This page contains some examples to teach you about the fundamentals of Deno.

This document assumes that you have some prior knowledge of JavaScript, especially about async/await. If you have no prior knowledge of JavaScript, you might want to follow a guide on the basics of JavaScript before attempting to start with Deno.

Hello World

Deno is a runtime for JavaScript/TypeScript which tries to be web compatible and use modern features wherever possible.

Browser compatibility means a Hello World program in Deno is the same as the one you can run in the browser.

Create file locally called first_steps.ts and copy and paste the code line below:

console.log("Welcome to Deno!");

Running Deno Programs

Now to run the program from the terminal:

deno run first_steps.ts

Deno also has the ability to execute scripts from URLs. Deno hosts a library of example code, one of which is a Hello World program. To run that hosted code, do:

deno run https://deno.land/std@0.103.0/examples/welcome.ts

Making an HTTP request

Many programs use HTTP requests to fetch data from a webserver. Let’s write a small program that fetches a file and prints its contents out to terminal. Just like in the browser you can use the web standard fetch API to make HTTP calls.

In the first_steps.ts file you created above, paste the code below:

const url = Deno.args[0];
const res = await fetch(url);

const body = new Uint8Array(await res.arrayBuffer());
await Deno.stdout.write(body);

Let’s walk through what this application does:

1. We get the first argument passed to the application, and store it in the url constant.
2. We make a request to the url specified, await the response, and store it in the res constant.
3. We parse the response body as an ArrayBuffer, await the response, and convert it into a Uint8Array to store in the body constant.
4. We write the contents of the body constant to stdout.

Try it out:

deno run first_steps.ts https://yirenlu.com/

or, from URL:

deno run https://deno.land/std@0.177.0/examples/curl.ts https://example.com

You will see this program returns an error regarding network access so what did we do wrong? You might remember from the introduction that Deno is a runtime which is secure by default. This means you need to explicitly give programs the permission to do certain ‘privileged’ actions, such as access the network.

Try it out again with the correct permission flag:

deno run --allow-net=yirenlu.com first_steps.ts https://yirenlu.com/

or, from URL:

deno run --allow-net=example.com https://deno.land/std@0.177.0/examples/curl.ts https://example.com

Reading a file

Deno also provides APIs which do not come from the web. These are all contained in the Deno global. You can find documentation for these built-in APIs here at /api.

Filesystem APIs for example do not have a web standard form, so Deno provides its own API.

In this program each command-line argument is assumed to be a filename, the file is opened, and printed to stdout.

const filenames = Deno.args;
for (const filename of filenames) {
  const file = await Deno.open(filename);
  await file.readable.pipeTo(Deno.stdout.writable);
}

The ReadableStream.pipeTo(writable) method here actually makes no more than the necessary kernel→userspace→kernel copies. That is, the same memory from which data is read from the file, is written to stdout. This illustrates a general design goal for I/O streams in Deno.

Again, here, we need to give –allow-read access to the program.

Try the program:

# macOS / Linux
deno run --allow-read https://deno.land/std@0.177.0/examples/cat.ts /etc/hosts

# Windows
deno run --allow-read https://deno.land/std@0.177.0/examples/cat.ts "C:\Windows\System32\Drivers\etc\hosts"

Putting it all together in an HTTP server

One of the most common usecases for Deno is building an HTTP Server.

http_server.ts

import { serve } from "https://deno.land/std@0.157.0/http/server.ts";

const handler = async (request: Request): Promise<Response> => {
  const resp = await fetch("https://api.github.com/users/denoland", {
    // The init object here has an headers object containing a
    // header that indicates what type of response we accept.
    // We're not specifying the method field since by default
    // fetch makes a GET request.
    headers: {
      accept: "application/json",
    },
  });

  return new Response(resp.body, {
    status: resp.status,
    headers: {
      "content-type": "application/json",
    },
  });
};

serve(handler);

Let’s walk through what this program does.

1. Import the http server from std/http (standard library)
2. HTTP servers need a handler function. This function is called for every request that comes in. It must return a Response. The handler function can be asynchronous (it may return a Promise).
3. Use fetch to fetch the url.
4. Return the GitHub response as a response to the handler.
5. Finally, to start the server on the default port, call serve with the handler.

Now run the server. Note that you need to give network permissions.

deno run --allow-net http_server.ts

With the server listening on port 8000, make a GET request to that endpoint.

curl http://localhost:8000

You will see a JSON response from the Deno GitHub page.

More examples

You can find more examples in the Examples chapter and at Deno by Example.