Bun is a modern JavaScript runtime designed to serve as a drop-in replacement for Node.js while also supporting browser-style web APIs. Its output format is built to support both browser and Node.js environments by combining web-standard APIs with Node.js compatibility features, enabling a unified development experience across these environments.
Bun's design incorporates the following key aspects to support both environments seamlessly:
1. Web-Standard APIs Integration: Bun is powered by Apple's JavaScriptCore engine (used in Safari) and implements a broad set of web-standard APIs such as `fetch`, `Headers`, `Request`, `Response`, `ReadableStream`, and `WebSocket`. These APIs are natively available in browsers and are also supported in Bun's runtime. This allows developers to write code that relies on these standard APIs and have it run consistently in both browsers and Bun without modification.
2. Node.js Compatibility: Despite its strong web-standards foundation, Bun is explicitly designed as a drop-in replacement for Node.js. It supports the Node.js module resolution algorithm, CommonJS modules, and many core Node.js modules like `fs`, `path`, `Buffer`, and `net`. Bun recognizes global variables used in Node.js such as `__dirname` and `process`, ensuring that existing Node.js codebases can run with minimal changes.
3. Module System and Transpilation: Bun supports both ECMAScript Modules (ESM) and CommonJS modules, facilitating compatibility with code written for Node.js and the browser. Additionally, Bun has built-in support for TypeScript and JSX, transpiling these files to JavaScript at runtime. This extends the common browser toolchain capabilities directly into the runtime and simplifies development workflows.
4. Bundler and Output: Bun includes a highly optimized bundler that can target output formats suitable for both Node.js and browsers. It bundles code by resolving dependencies, transpiling TypeScript/JSX, and producing output that works immediately in the intended environment. The bundler supports minification and optimization strategies tailored for each context â for instance, swapping out Node.js-specific modules for browser-compatible versions where needed.
5. Unified Development Experience: Because Bun's APIs and runtime environment blend Web API compatibility and Node.js features, developers can write code once and run it both server-side and client-side. This reduces context switching and decreases the need for environment-specific shims or polyfills. For example, the use of the `fetch` API looks the same whether code runs in the browser or Node.js under Bun.
6. Performance Optimizations: Bun's runtime execution is optimized for speed, with module resolution and file operations (like reading and writing files) being much faster than Node.js equivalents. This performance gain applies across both environments without sacrificing compatibility.
7. Environment Detection and Adaptation: While Bun provides a common runtime, the developer can still write environment-specific code paths when needed by checking built-in globals (e.g., `process.browser` or similar flags). The output format and bundler support conditional imports and dynamic code loading to adapt to the environment at runtime, thus supporting complex universal JavaScript applications.
In summary, Bun's output format supports browser and Node.js environments by implementing web-standard APIs alongside Node.js compatibility layers, supporting ESM and CommonJS modules, and offering built-in transpilation and bundling to produce optimized code for either environment. Its runtime and toolchain are unified to allow seamless execution of the same codebase across client and server, leveraging both browser features and Node.js expectations simultaneously. This design creates a highly versatile and performance-optimized JavaScript runtime environment suitable for modern full-stack development.