Hello Deno!

If you are familiar with NodeJS, you may be interested in learning about Deno, a new JavaScript runtime built by Ryan Dahl, the creator of Node.js. Deno is built on the V8 JavaScript engine but was built using Rust instead of C++, and one of its main goals is to bring server-side JavaScript closer to browser JavaScript. In this article, we will introduce you to Deno and its features.

Importing Packages

Deno uses URLs to import packages, and it caches them locally on the first run, similar to npm install. For example, to import the Oak web framework, you can use the following code:

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

Deno doesn't need to npm install packages. A similar effect can be achieved with deno cache --node-modules-dir <glob>

Bundling and Compiling

Deno has built-in tools for bundling and compiling TypeScript code. To bundle your code, you can use the following command:

# deno bundle <in> <out>
$ deno bundle app.ts app.bundle.js

To compile your TypeScript code, you can use the following command: This will build a self contained executable.

# deno compile <flags> <input>
$ deno compile --allow-net app.ts

Both bundling and compiling to standalone executable is a built-in feature of Deno.

Configuring Deno

Deno uses deno.json to specify configuration options for your application. This file is similar to package.json's scripts section in NodeJS. You can use it to specify command-line options like this:

{
  "scripts": {
    "start": "deno run --allow-net app.ts"
  }
}

The documentation has detailed sections on:

• Configure the TypeScript compiler options
• linting
• formatting
• imports (alternative to import_map)
• locking
• nodeModulesDir

Managing dependencies

There are two main ways of collecting and managing dependencies. Either by putting them all in deps.ts or use an import_map either through an import_map.json or deno.json's imports property

deps.ts

By convention in Deno land we use a file called deps.ts to manage dependencies. This file is similar to package.json in NodeJS and contains all the dependencies required by your application. You can import dependencies from deps.ts like this:

deps.ts

export { Application } from "https://deno.land/x/oak@v12.6.0/mod.ts";

main.ts

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

deno.json or import_map.json

Deno uses import_map.json to map URLs to local file paths. This file is similar to package.json's dependencies section in NodeJS. You can use it to map URLs to local files like this:

Using import_map.json

import_map.json

{
  "imports": {
    "oak": "https://deno.land/x/oak/mod.ts"
  }
}

deno.json

{
  ...
  "importMap": "./import_map.json"
}

Using only deno.json

deno.json

{
  "imports": {
    "oak": "https://deno.land/x/oak/mod.ts"
  }
}

Now importing oak works like you're used to in NodeJS, with simply using a naked specifier and not a full blown URL. This is desirable when you have to import a package in multiple places or migrating a NodeJS project to Deno.

Pro tip: If you have a package.json in your project, deno can detect it and us it as an import map.

import { Application } from "oak";

Built-in Tools

Deno comes with several built-in tools, including a test runner, a code formatter, and a linter. These tools can be used to improve the quality of your code and make development easier.

Web Standard Compatibility

Deno aims to have the same APIs on the server that you would use in the browser. This means that you can use web standards like the fetch API on the server, making it easier to write code that works in both environments.

Locking and Vendoring

Deno does not have a built-in package manager like npm, but it does have a mechanism for locking and vendoring dependencies. You can use the --lock and --lock-write options to lock your dependencies, and the --import-map option to vendor them.

NPM Support

Deno does support npm packages with the special npm: modifier import or through esm transpilers like esm.sh;

¹ Deno does not have a direct equivalent for installing local packages or dependencies.

² deno check is similar to TypeScript's tsc but not a one-to-one replacement.

Conclusion

In conclusion, Deno is a new JavaScript runtime that aims to bring server-side JavaScript closer to browser JavaScript. It has several features that make it a compelling alternative to NodeJS, including built-in tools, web standard compatibility, and a mechanism for locking and vendoring dependencies. While Deno is still relatively new, it is worth exploring for your next project.