JavaScript Runtimes

A runtime environment is the system where your JavaScript code actually runs. The runtime you choose determines several critical factors about how your application operates:

1. Available APIs: Does your code have access to browser-specific features (like fetch or the canvas), or server-specific features (like file system access)?
2. Execution Method: How your code is compiled and interpreted (e.g., JIT compilation).
3. Dependencies: What external packages you need to install in production.
4. Environment: Whether your code runs on the frontend (a browser/mobile app) or the backend (a server).

Common Runtimes

Originally, JavaScript was designed to run exclusively in web browsers. Today, thanks to the explosion of modern runtimes, it runs almost everywhere.

• The Browser: Chrome, Firefox, Safari. (While we treat “the browser” as one concept, each engine—like V8 or SpiderMonkey—is technically its own runtime).
• Node.js: The oldest and most popular backend runtime for running JavaScript on servers.
• Deno.js: A modern backend runtime built with security and native TypeScript support in mind.
• Bun: An extremely fast, modern all-in-one toolkit and runtime.

If you understand core JavaScript, you can easily work within any of these runtimes. The language syntax remains exactly the same; the only difference is the specific APIs the runtime provides you.