Deno - Modern JavaScript Runtime

What Deno Actually Is

Deno is a JavaScript and TypeScript runtime built on V8 and Rust.

Ryan Dahl created it after getting frustrated with Node.js's design decisions. Deno 2.0 launched in October 2024 and it's finally stable enough that you won't get weird crashes every other day.

The whole point is security by default. Unlike Node.js, which gives scripts full system access unless restricted, Deno blocks everything. Want to read files? You need --allow-read. Network access? That's --allow-net.

I had a malware package try to phone home during development once

• the script just crashed with a permission error instead of silently stealing my AWS keys. That's when I realized this security model actually works.

TypeScript Without Configuration Hell

Deno runs .ts files directly. No tsconfig.json, no build step, no transpiler setup bullshit. You write TypeScript, you run it. This eliminates about 90% of the "it works on my machine" issues that make you want to throw your laptop out the window.

Testing is built-in too. deno test runs your tests without installing Jest, Mocha, or whatever testing framework the JavaScript community decided to obsess over this month.

npm Compatibility

Deno 2.0 finally imports npm packages directly:

import express from \"npm:express@4.18.2\";

Most packages work fine. Express, lodash, axios

• the usual suspects work without drama. But good luck with anything that makes Node.js-specific filesystem assumptions or uses native modules. I spent 3 hours debugging a package that expected __dirname to exist.

Performance

Deno performs about the same as Node.js for most workloads. Both use V8, so JavaScript execution is identical. The Rust implementation makes some system operations faster, but for typical web applications you won't notice the difference.

Cold starts are faster than Node.js, which actually matters for serverless deployments where you're not keeping processes warm.

The Downsides

The ecosystem is tiny compared to npm. While npm compatibility helps, you're still dealing with a platform that has maybe 1% of Node's package ecosystem. Some advanced Node.js features just don't exist yet.

The permission flags get old fast. You'll be typing --allow-net --allow-read --allow-write --allow-env so often you'll want to make an alias. Most people just use --allow-all in dev because life's too short to remember which flag lets you read a config file.

TypeScript support, while excellent, can be slower for large projects since there's no incremental compilation yet. On a 50k+ line codebase, you'll notice the compile time difference compared to tsc --watch. It's not dealbreaker slow, but it's noticeable.

The Package Situation

JSR (JavaScript Registry) is Deno's package registry focused on TypeScript-first packages. It's got decent TypeScript support with automatic documentation generation. As of September 2025, it contains several thousand packages but that's still a rounding error compared to npm's millions.

For npm compatibility, Deno 2.0 can import packages directly:

import express from "npm:express@4.18.2";
import lodash from "npm:lodash";

Most common packages work fine. Express, MongoDB drivers, utility libraries like lodash generally work without drama. Problems arise with packages that make Node.js-specific filesystem assumptions or use native modules. Spent a weekend migrating a project and hit 12 different compatibility issues with various npm packages.

Deployment Options

Deno Deploy is their serverless platform. It's similar to Vercel or Netlify Edge Functions but built specifically for Deno. Cold starts are fast, the developer experience is decent, and it handles WebSocket connections without the usual serverless limitations.

For traditional hosting, Deno compiles to a single executable that includes your code and dependencies. This makes containerization way simpler than Node.js projects - no node_modules copying, no npm install steps that take forever in Docker builds.

Web Frameworks

Fresh is the main full-stack framework, built on Preact with an "islands architecture" where only interactive components send JavaScript to browsers. It's fast and lightweight but has a tiny ecosystem compared to Next.js - good luck finding that specific middleware you need.

For API development, most Express-style frameworks work through npm compatibility, though some middleware may have compatibility issues.

Real-World Usage

Companies using Deno in production include Slack for internal tools, Netlify for edge functions, and Supabase for database functions. These are mostly for newer services or internal tooling rather than core business logic, which tells you everything about how companies really feel about adopting it for critical systems.

How Fast It Is

Deno performs similarly to Node.js for most applications. Both use V8, so JavaScript execution speed is identical. Deno's Rust foundation makes some system operations faster, while Node.js has optimizations from years of production use and thousands of engineers beating on it.

For I/O-intensive applications (APIs, web servers), the performance difference is negligible. For CPU-intensive tasks, both are equally limited by V8's JavaScript performance.

Moving From Node.js

Moving from Node.js requires rewriting import statements, dealing with permission flags that will drive you insane, and potentially replacing packages that don't work with npm compatibility. Simple applications migrate easily - I moved a basic Express API in about 2 hours. But complex projects with many dependencies? Budget 2-3x longer than you think, minimum.