Nostalgist.js is a TypeScript library that provides a simplified API for running retro console emulators in web browsers using RetroArch compiled via Emscripten. It handles the complex setup of Emscripten modules, virtual file systems, and RetroArch configuration, allowing developers to launch games with minimal code. The library supports a wide range of consoles (NES, SNES, Game Boy, etc.) with sensible defaults for loading cores and ROMs from CDNs, while also allowing custom resolvers. It includes features like save/load states, SRAM management, screenshot capture, and programmatic input control. The codebase is well-structured with strong TypeScript types, comprehensive tests (integration and e2e), and a clean class hierarchy. Notable highlights include a flexible file resolution system that handles various input formats (URLs, Blobs, ArrayBuffers, etc.) and a clever patching mechanism for importing Emscripten modules as ES modules.