butterchurn is a browser-based, real-time music visualizer that faithfully reimplements the classic MilkDrop visualizer using WebGL2. It consumes audio from the Web Audio API, runs FFT analysis to derive frequency/amplitude data, and drives a shader-based render pipeline from JSON presets that encode MilkDrop-style equations. The core library exposes a minimal `createVisualizer()` entry point, after which consumers load a preset and call `render()` with audio level data each animation frame. The library supports two equation execution paths: a JS-based expression runner and a WebAssembly/AssemblyScript-based EEL runner, with a custom Rollup plugin that compiles AssemblyScript to base64-encoded WASM at build time. It also includes deterministic seeded RNG for reproducible rendering, which is used by its visual regression test suite to compare JS and WASM paths pixel-for-pixel across a curated set of presets.