esm.sh is a proxy-based registry and distribution system for buildless web development. It retrieves packages from external registries and serves them as ES modules directly to the browser, eliminating the need for a local build step.
The project transforms TypeScript, JSX, TSX, and Vue files into browser-compatible JavaScript on the fly during the request process. It handles Node.js built-in resolution by mapping internal modules to browser-compatible polyfills and manages dependency versioning and aliasing through URL query parameters.
The system optimizes delivery via request-time tree shaking to remove unused code and can bundle a module with its external dependencies into a single file. Additionally, it provides mechanisms for bootstrapping and executing JavaScript modules within a web worker context.