From Browsers to Bun: A Beginner’s Guide to JavaScript & TypeScript Runtimes

When you write code in JavaScript or TypeScript, your computer doesn’t magically understand it. It needs something in between — a runtime environment.

Think of a runtime as the stage where your code performs, with all the props, lights, and backstage crew ready to make it work.

If you’ve ever heard terms like Node.js, Deno, Bun, or run your code in a browser, you’ve already brushed against different runtimes — even if you didn’t know it.

Let’s break it down step-by-step.

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1. What is a Runtime Environment?

A runtime environment is the combination of:

• Engine - the part that executes your code (e.g. Google’s V8, Mozilla’s SpiderMonkey).

• APIs & Tools - built-in features that aren’t part of the language itself. (e.g. fetch() , file system access).

• Event Loop & Scheduler - the system that handles asynchronous tasks (timers, network requests, etc.).

Without runtime, JavaScript is just a plain text — it can’t “run” on its own.

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2. Why Do We Need Different Runtimes?

Different environments exist because JavaScript isn’t tied to one platform anymore.

Originally, JavaScript ran only in browsers for web interactivity. Today, it powers:

• Servers (Node.js)

• Command-in tools

• Desktop & mobile apps

• IoT devices

Each use cases demands different built-in features and optimizations, which is why we have multiple runtimes.

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3. Popular JavaScript & TypeScript Runtime Environments

a) Browser (Chrome, Firefox, Safari, Edge)

• Engine:

  • Chrome & Edge → V8

  • Firefox → SpiderMonkey

  • Safari → JavaScriptCore

• Best for: Running code that interacts with web pages.

• APIs available: DOM, fetch(), localStorage, WebSockets.

• Limitations: No direct file system or server-side access (for security).

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b) Node.js

• Released: 2009

• Engine: Google’s V8 (same as Chrome)

• Best for: Server-side applications, CLI tools, backend APIs.

• APIs available: File system (fs), HTTP server, process management.

• TypeScript support: Requires a transpiler like tsc or bundlers like esbuild.

Key advantage: Huge package ecosystem via npm.

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c) Deno

• Released: 2018 (by Node.js creator Ryan Dahl)

• Engine: V8 + Rust-based bindings.

• Best for: Secure, modern apps with TypeScript support out of the box.

• APIs available: Web-standard APIs, file system (with permission flags), fetch().

• TypeScript support: Native — no seperate compiler needed.

Key advantage: Secure by default, no node_modules, modern ES modules.

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d) Bun

• Released: 2022

• Engine: JavaScriptCore (from Safari)

• Best for: Extremely fast dev environments, full-stack apps.

• APIs available: Node.js-compatible APIs + Web APIs.

• TypeScript support: Built-in.

Key advantage: Focus on speed — fast startup, bundling, and package installs.

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4. Quick Comparison Table

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5. How to Choose the Right Runtime

• Building a website UI? → Browser

• Backend API or CLI tool? → Node.js or Bun

• Want secure defaults & native TypeScript? → Deno

• Need speed & modern tooling? → Bun

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6. Final Thoughts

JavaScript and TypeScript are language specs — they need a runtime environment to actually do anything. Browsers, Node.js, Bun and Deno are just different stages for the same actors (your code). Choosing the right one depends on where and how you want your code to perform.

If you’re just starting out:

1. Learn JavaScript in the browser (like original playground)

2. Try Node.js for backend projects

3. Experiment with Deno and Bun to see what modern runtimes offer.

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💡 Pro Tip: Even though each runtime feels different, they all share the same core JavaScript fundamentals. Master those, and you can work anywhere.