A Quick Guide to Deno

I've been using Deno for a while now and I am really enjoying it.

This article is actually created based on a talk I did, so just keep that in mind for context.

The Folly of Man

Why NodeJS is terrible

• - Run Javascript on the server!
• - Tired of having a fast and secure server environment? We have the solution!
• - Make sure to use proprietary module loading formats for extra confusion!

const fs = require("fs"); // What is this?

• - Cross-compatibility? What’s that? We load weird binaries constantly with .node files!
• - :node_modules:

That was dumb

• - Run, Bundle, and Compile Typescript (or Javascript).
• - Security is built-in. Everything is sandboxed.
• - Uses official ECMAscript syntax for imports.
• - WebAssembly for modules with speed (no proprietary formats)
• - Caches all modules in one centralized location. No more gigabytes of downloads per project.

Install

• - Installing Deno does not require a custom installer because it’s literally just one executable.

Quick Install Script:

curl -fsSL https://deno.land/x/install/install.sh | sh

Breakdown (for those of us who aren't linux experts):

• - curl: download script from URL
• - |: pipe the output to the next program
• - sh: run a bash script

Overview

Deno namespace

• - If you want to do something closer to system-level, Deno provides a number of APIs through the “Deno” namespace

Deno.env.get("TMPDIR"); // get environment variable

Deno built-in library

• - Deno, unlike Node does not treat built-in modules any differently than anything else.
• - Deno does provide modules built by the team publicly but it is referred to as the “Standard Library” (presumably to make it more similar to other better languages)
• - Deno STD isn’t stable yet
  • - This doesn’t mean it can’t be used in production, it just means functionality/existence of a feature might change in future versions
• - Deno’s STD is heavily inspired by Go’s which makes it very complete
• - At the time of writing, it is currently at v0.140.0

Deno module resolution

• - Modules are defined by links pointing to js or ts files.

import { Application } from "https://deno.land/x/oak/mod.ts";

• - The first time a module is used, it is fetched and compiled by Deno and is then never re-downloaded again.
  • - This means that if the web server hosting the module goes down, your code still works.
• - Integrity checking is important.
  • - Deno supports and encourages using a lock file (a json with links and the hash of the file contents)

// lock.json
{
  "https://deno.land/std@0.139.0/textproto/mod.ts": "3118d7a42c03c242c5a49c2ad91c8396110e14acca1324e7aaefd31a999b71a4",

  "https://deno.land/std@0.139.0/io/util.ts": "ae133d310a0fdcf298cea7bc09a599c49acb616d34e148e263bcb02976f80dee",

  "https://deno.land/std@0.139.0/async/delay.ts": "35957d585a6e3dd87706858fb1d6b551cb278271b03f52c5a2cb70e65e00c26a"
}

Okay but how do I use them in my code?

“URL imports are a nightmare for version control” - Matt Two main methods

• - dep.ts
  • - Create a typescript file that imports and re-exports only the parts that you need

// dep.ts
export { Application } from "https://deno.land/x/oak/mod.ts";

// main.ts
import { Application } from "./dep.ts";
// ...

• - import_map.jsonc
  • - Store all of your dependencies in an import map, import modules from that path

// import_map.jsonc
{
	"imports": {
		"oak/": "https://deno.land/x/oak/"
	}
}

// main.ts
import { Application } from 'oak/mod.ts'
// ...

Quick Note: In both cases you would want to version your module:

import { Application } from "https://deno.land/x/oak@10.5.1/";

Testing in Deno

No more third-party frameworks for testing!!!

• - Just run deno test

import { assertEquals } from "https://deno.land/std@0.140.0/testing/asserts.ts";

Deno.test(() => {
  assertEquals("monkey", "monkey"); // success!
});

Linting/Formatting in Deno

It’s built in. Finally.

• - Use the deno lint and deno fmt commands to do this.
• - The defaults are pretty good, and are probably what you want anyways.
  • - Personally, I always hated setting up eslint and finding which rules I liked best.
  • - Customizable with the deno.json file.

Bundling in Deno

Guess what? It’s built in.

• - NO MORE WEBPACK!
• - Just run deno bundle
  • - By default this will just output the result in stdout, you can specify a file name if you want it to be saved. Otherwise, you could pipe the result somewhere else (maybe to something like babel).

Compiling in Deno

What?

• - You can compile your Deno utilities straight down to an executable. This is more of a cool feature than something particularly useful but why not

Common Doubts

“Cool, but useless with an Ecosystem”

• - Usually this would be a big concern for a new runtime since usually it implies a new language.
• - Deno just runs typescript (or javascript), the code is already out there!
• - You want to use a node module in deno? You have two options:
  • - Skypack
    • - Allows you to import basically all ESM node modules just like any other file!
  • - DNT
    • - A command-line tool that allows you to transform a node module into a deno module by some magic and inserting shims when necessary
• - If you prefer the workflow of Deno, NOTHING IS STOPPING YOU FROM USING IT NOW

“It doesn’t really run Typescript, it’s lying”

• - This is fundamentally just nitpicking but I’ll elaborate anyways. I think quoting the manual explains it best.

“But wait a minute, does Deno really run TypeScript? you might be asking yourself. Well, depends on what you mean by run. One could argue that in a browser you don't actually run JavaScript either. The JavaScript engine in the browser translates the JavaScript to a series of operation codes, which it then executes in a sandbox”

• - Deno uses a combination of TSC and SWC to translate your code to javascript in real time.
  • - Tip: Type checking can be slow, so if you want to avoid waiting you could just use the --no-check flag in dev and remove it for tests in prod.

“Okay but I use X obscure feature with Typescript. Now what?”

• - You can configure Typescript using a tsconfig file like any other system.
  • - Tip: You can also use deno.json to customize your typescript!

“How is the module resolution any better than Node”

• - This is mostly a matter of perspective. “Better” is a judgement.
• - In my opinion, URL imports are a significantly better idea for any developer who supports freedom because it removes the need for centralized package managers.
  • - Package managers do exist if you really want them, but personally I do not recommend using them with Deno
• - You don’t need to download the module every time you need it. If it’s the same module as before, why waste your precious disk space.
• - The lock file REALLY is a lock file with no magic upgrading of modules.

“URL imports are bad because what if host server goes down”

• - True.
• - How is this any different than NPM? Or Rust Crates? Or literally anything on the internet?
• - You can mitigate the dangers of this by using some stuff we’ve already talked about
  • - Use a lock file. Make sure that if the web-server somehow gets hacked, you’ll be fine.
• - Mission-critical programs already have a solution to this. You have two options:
  • - Bundle your program using built-in tools to remove need for this
  • - Simply put the modules in your version control. Yes, Deno does support this.

“I miss my package.json because I want my npm scripts”

• - Deno has a configuration file (which should be named deno.json or deno.json5) that allows for a ton of tweaking with options.
  • - You explicitly do NOT need it. Everything that you could want to do can be done by passing in arguments
• - Deno.json has a “task” system very similar to npm scripts. Just run deno task x

{
 "tasks": {
   "test": "deno test",
   "giveup": "rm -rf /*”
 }
}

Conclusion

I believe that Deno has a good chance of making it big, especially in enterprise with it's focus on security, reliability, and customer support. While I don't think Deno will ever really replace Node, I truly think it has a good shot of being a popular alternative.

If you have different opinions, let me know! I'd love to have a dialogue and hear your perspective.

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