It's a DAG-computing framework in JS that allows you to listen to updates in a compute graph. It's a set of legos that I used to build both a MobX clone (https://github.com/atlassubbed/atlas-munchlax) and a React clone (https://github.com/atlassubbed/atlas-mini-dom). React and MobX are actually the same thing mathematically: reactive DAGs, so they can both be built using the exact same abstraction.