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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.