JavaScript Runtimes Compared - Browsers vs Node vs Deno vs Bun

JavaScript is a versatile language that can run in different runtime environments, each with its own capabilities. In this guide, we'll compare popular JavaScript runtimes including browsers, Node.js, Deno, and Bun in-depth.

Browser Runtimes

The browser provides the original and most widely used runtime for JavaScript. Its Key characteristics are:

• Runs client-side JS only - no access to servers, files etc.

• Provides full DOM API access for interacting with web pages.

• Web APIs available like Fetch, LocalStorage, Service Workers, WebAssembly and more.

• JavaScript executes in a single thread shared with other resources. Concurrency is achieved via event loop.

• Browser runtimes sandbox code execution for security. Restricts filesystem/network access.

• Each browser engine like V8 (Chrome), SpiderMonkey (Firefox),JavaScriptCore (Safari), etc; provides its own implementation.

Browsers offer the most mature and fully-featured runtime optimized for client-side web development.

Node.js Runtime

Node.js introduced the ability to run JavaScript on the server. Its Key properties are:

• Runs JS on a server outside a web browser.

• Provides APIs for the filesystem, networking, HTTP servers, child processes etc.

• Still a single-threaded event loop model but with more control over concurrency.

• Large ecosystem of 3rd party modules available via NPM.

• Built on Google's V8 engine also used in Chrome.

• Offers CommonJS module system compared to ES Modules in the browser.

• No browser Web APIs but can render React server-side.

• Used heavily for backend web services and tooling.

Node.js opened up serverside development use cases for JavaScript.

Deno Runtime

Deno is a newer runtime created by Node's inventor focused on security and extensibility. Its key features are:

• Aims to fix fundamental issues in Node's design like security.

• Secure by default. No file, network, or environment access unless explicitly enabled.

• Ships only a few stdlib modules. Most features are provided via URLs importing modules.

• Supports the latest standards like ES Modules and TypeScript.

• Includes browser-compatible APIs like Fetch, Timers, and Files.

• Designed to interoperate with browser runtimes.

• Still a single-threaded event loop model but with more modern async handling.

• Third-party modules are imported via URLs from any host rather than the central registry.

Deno offers a more secure and standards-compliant alternative to Node's capabilities.

Bun Runtime

Bun is a new runtime focused on fast startup times and bundling. Its key features are:

• Built-in Rust for high performance. Quick startup and execution.

• Includes only essentials for bundling like ES Modules, import maps etc.

• Aims to replace complex bundlers like Webpack and Vite.

• No filesystem access by default. Sandboxed like browsers.

• Can run in browsers and Node context. Isomorphic.

• Supports JSX, TypeScript, CSS imports etc out of the box.

• Third-party modules are imported and bundled via URLs.

• Very limited runtime APIs. Focused on bundling output.

Bun optimizes JS bundling and runtime performance rather than API surface.

Summary

• Browsers offer the most mature and extensive environment for client-side JS.

• Node.js enables server-side JS for backend services and tooling.

• Deno provides updated server-side runtime focused on security and standards.

• Bun runtime specializes in optimized JS bundling performance.

Each environment brings specific benefits depending on the use case needs.