What is Deno?

Deno 1.0, a runtime for JavaScript and TypeScript, rolled out in May with appealing features for JavaScript developers, including:

• Secure defaults: Explicit permission must be granted for your Deno applications in order to access disk, network, and runtime environments.
• Native TypeScript support: No tsconfig needed—Deno acts like a native TypeScript runtime. Under the hood Deno still transpiles and bundles TypeScript files.
• Ability to import ES Modules directly from URLs: No more NPM—dependencies can be imported directly via URL or file path:

import toUpperCase from 'https://flavio-es-modules-example.glitch.me/uppercase.js';

Deno’s approach to ES Modules is generating a lot of debate around package management, especially concerning security. For example, will this prevent another left-pad incident? Regardless of your gut reaction, I highly recommend reading the docs.

I think the explicitness of import-from-URL will make developers think carefully about dependency management; however, I suspect many teams will handle this problem similarly to how they handle npm: with lock files, proxies, and allow-listed internal registries.

Deno also includes less-cited features that I’m excited about:

• Bundler: Deno can bundle your application and its dependencies into a single file.
• Script Installer: Deno can install a script from any URL, add its command-line alias to PATH, and requires explicit permissions during installation. Wow!
• Async Standard Library: Deno standard APIs use async/await and async iterators.
• Built-in test framework: Deno includes a built-in test framework with top-level async support.

Enough talk, let’s take Deno for a spin by building a simple web app:

import { serve } from "https://deno.land/std@0.50.0/http/server.ts";

const s = serve({ port: 8000 });
console.log("http://localhost:8000/");
for await (const req of s) {
  if (req.url.startsWith("/ping")) {
    req.respond({ status: 200, body: "pong" });
  } else if (req.url.startsWith("/hi")) {
    req.respond({ status: 200, body: "hello there." });
  } else {
    req.respond({ status: 404, body: "Not Found." });
  }
}

This demo uses the HTTP standard library and handles two routes: /ping and /hi. Note the use of for await … of without an enclosing async IIFE. Install Deno, then run:

# run locally
deno run demo.ts 

# run from remote URL
deno run https://gist.githubusercontent.com/robzhu/ef4e76d27ca50a920a9e5b4aebb40ab9/raw/6bef3fa5eb6bca5f439c3fde75ce2e39cf4cd451/demo.ts

Either way, you should see an error:

error: Uncaught PermissionDenied: network access to "0.0.0.0:8000", 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)
    at serve (https://deno.land/std@0.50.0/http/server.ts:261:20)
    at https://gist.githubusercontent.com/robzhu/ef4e76d27ca50a920a9e5b4aebb40ab9/raw/6bef3fa5eb6bca5f439c3fde75ce2e39cf4cd451/demo.ts:3:11

Since this is an HTTP server, we need to grant explicit permissions for network access:

# run locally
deno run --allow-net demo.ts 

# run from remote URL
deno run --allow-net https://gist.githubusercontent.com/robzhu/ef4e76d27ca50a920a9e5b4aebb40ab9/raw/6bef3fa5eb6bca5f439c3fde75ce2e39cf4cd451/demo.ts

Now you should be able to test your server by navigating to http://localhost:8000/hi. So far so good, but that URL is quite long: Is there an easy way to install it as a script?

deno install --allow-net -n denohttp https://gist.githubusercontent.com/robzhu/ef4e76d27ca50a920a9e5b4aebb40ab9/raw/6bef3fa5eb6bca5f439c3fde75ce2e39cf4cd451/demo.ts

# run the script:
denohttp

I hope this post has given you an overview of Deno.

Deno is released under the MIT License. Learn more in the Deno GitHub repository or visit the Deno website.