I recently published a book, Deno Web Development

learning as we go

Adventures in Deno land 🦕

8 min read - 2020-05-15

Earlier this week Deno was released.

As I was very excited ever since I first heard about it on Ryan Dahl’s talk at jsconf, I had to give it a try.

This talk is one of my personal favorites, it is a lesson on humility. Having Ryan looking at what he built 10 years ago with a criticizing tone is interesting. Even when node is used by millions of people, its creator still feels bad about some decisions made at the time.

Getting back to what brought me here… After hearing of the launch of v1.0 I took some hours to learn more about it. The documentation is very well written and structured, which by following what they call the manual one could have a very good understanding of how to start using it.

Building something

After reading the documentation, it looked great, in theory. But my default way to learn is normally to build something with it. It normally helps me identify pains I’d have in the real world if I had to build a real application with it.

The decision was to build an API that connects to twitter and returns 15 tweets from a user with more than 5 likes, I called it popular tweets. This small server should then run on a Kubernetes environment.

If you wanna follow the code, here you have it

At first, I was kinda lost and didn’t know any APIs. I’ve decided to go explore the standard library. I got very impressed by how approachable was the code, took some time to read it, and learned a ton.

It got this idea on the back of my mind, which might lead to a future article, similar to what Paul Irish did 10 years ago on 10 things I learned from the jquery source but for Deno source, might actually do it!

After getting to know the basics, installing the VSCode plugin and Deno, we were ready to start my adventure.

To be honest, it wasn’t a real adventure, everything looked so familiar that I almost forgot I was using a different runtime.

Getting to code

By using the standard library’s http server it was very easy to build a server and get it up running handling requests.

import { serve } from "./deps.ts";

const s = serve({ port: 8080 });

for await (const req of s) {
    status: 200,
    body: "Hello world",

Step 2 was to connect it to twitter API. Having fetch already included on deno made it very easy and familiar.

  "https://api.twitter.com/1.1/search/tweets.json&q=(from: ampsantos0 min_faves: 5)",
  { headers: new Headers([["content-type", "application/json"]]) }

Deno opted for mimicking existing Web APIs where they existed, rather than inventing new proprietary ones. For APIs that are not web standard, the Deno namespace is used. This looks like a smart choice to me, improving discoverability and reusing knowledge developers already have of the existing APIs.

Running it

Running the code was a breeze. One of deno’s selling points is security and I couldn’t agree more, it improved over node. You notice it the first time you try to run a program:

$ deno run ./index.ts

Once we are, in this case, using network to both expose our endpoint (:8080) and access Twitter’s API without our explicit consent, here’s what you get:

error: Uncaught PermissionDenied: network access to "", run again with the --allow-net flag
    at unwrapResponse ($deno$/ops/dispatch_json.ts:43:11)
    at Object.sendSync ($deno$/ops/dispatch_json.ts:72:10)
    at Object.listen ($deno$/ops/net.ts:51:10)
    at listen ($deno$/net.ts:152:22)

This is a very reasonable and comprehensive error, again, good job on this!

A good approach for this is by enabling whitelist permissions by using the flag --allow-net which Deno does it in a very simple and pragmatic way:

$ deno run --allow-net=,api.twitter.com index.ts

When running the code, the --inspect flag enables developers to use Chrome Dev Tools the same way they did in node, the debugging experience is as good as developers are used to.

Module resolution

When Ryan first talked about Deno, and the mistakes made in node’s design, one of the big things he mentioned that node’s way of importing modules was too complicated and had lots of edge cases.


const path = require("path")

The dependency we’re importing, path might come from node standard library. At the same time, it can come from node-modules, or you could have installed a dependency named path, right? Ok, now you found the dependency, do you know what is the file you are requiring? Is it index.js? What if package.json has a different main file defined?

Lots of unknowns…

What about local imports? When you do:

const add1 = require("./utils/math")

Is math a file? Or a folder with an index.js inside of it? What is the file extension? Is it .js, .ts?

You get the point… Node imports are hard.

Deno follows a golang like approach, of having absolute urls. If it sounds strange to you, bare with me. Let’s look at the advantages:

  • It solves local imports by adding the extension to it.
import { add1 } from "./utils/math.ts"

You know just from reading it that math.ts is a file.

  • It solves third party imports by having an absolute URL
import { serve } from "https://deno.land/std/http/server.ts"

No more magic module resolution.

This absolute module resolution enabled some fun stuff like what R. Alex Anderson did, running code from a set of gists.

Note: VSCode plugin functions well with the third party imports, you can cmd+click on dependency and you’re directed to the code, as usual.

Keeping track of dependencies

Let’s talk about managing dependencies. As deno simplified the module imports, it allowed it to automatically cache dependencies.

When you first try to run it, it downloads the dependencies, caches them, and then runs with the cached version.

To force the caching of a module without running it, you can run $ deno cache [module url].

You are probably thinking it is strange and error-prone to URLs all around the code? That’s right. You can manage it however you want, as all modules have absolute URLs now, it’s just code at the end of the day.

Deno recommends having a deps.ts file, you can call it whatever you want but since it is in the documentation, I see this start becoming a standard. On that file, you can import all the dependencies from the URLs and export the methods used.

// deps.ts
export { serve } from "https://deno.land/std/http/server.ts"
export { parseDate } from "https://deno.land/std/datetime/mod.ts"

// index.ts
import { serve } from "./deps.ts"

Having one single deps.ts file allows you to do some caching (as you did in package.json) on docker builds.

COPY deps.ts .
RUN deno cache deps.ts

By doing this, the RUN command will only run if the deps.ts file changed. With this, and as the installation step is now automatic, running it on docker became simpler.

There is one thing that has to be taken care of with Deno, we have to send the flags for the permissions.

CMD ["run", "--allow-net", "index.ts"]

Deno binaries

Deno provides an install command. But, as I said earlier, it does not install dependencies on the project, as that is done automatically.

Its usage is similar to the npm install --global flag, citing the explanation on the official website about install:

This command creates a thin, executable shell script which invokes Deno using the specified CLI flags and main module. It is placed in the installation root’s bin directory.

When you install a global binary, you have to specify what permissions it will need to run, again, secure by default.

$ deno install --allow-net --allow-read https://deno.land/std/http/file_server.ts

And you can then run $ file_server


Coming from the JS/TS world I’d say Deno got lots of things right. It has the familiarity of JS and TS with small twists, to the better side. Having the standard library written in TS is a big plus, at it isn’t always straightforward to set it up in node.

The standard library is great, it looks both readable and well thought. Quoting the deno_std main repo:

deno_std is a loose port of Go’s standard library. When in doubt, simply port Go’s source code, documentation, and tests.

This is funny and interesting at the same time, Deno used the effort the golang community put in its standard lib to drive its own, the result looks great.

The permission system is great and intuitive. Module resolution is now simpler and removes pretty much all the magic we got used to in node lands. All the async APIs return Promises now. It means using await and .then everywhere, not incurring into callback hell and not needing tools like promisify and such.

Adding to all of this, Deno also got inspiration from golang by shipping a lot of the essential tools in the main binary. Discussions about bundler, formatter, and test runner will no longer be a thing, and even if they are, there’s an official way now. I haven’t tried the test suite and the documentation generator yet, I might write about it later.

Gotta say the overall experience of building a (very small) application with it was very good and intuitive. Can’t wait to build something more real with it!

I’m excited to see how this will evolve and thus I wrote another article, my second adventure in Deno land where I go a little deeper on the runtime.

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