MobX is a production-grade, transparent reactive state management library for JavaScript and TypeScript, widely regarded for its minimal boilerplate and automatic dependency tracking. The monorepo contains the core reactivity engine (observable, computed, action, reaction, flow), official React bindings (mobx-react-lite for functional components, mobx-react for class components), an ESLint plugin enforcing best practices, and a codemod tool (mobx-undecorate) to migrate legacy decorator syntax to the modern makeObservable API. The codebase is architected around a small set of well-defined primitives: observable values (via Proxy for arrays, objects, and dedicated classes for maps/sets), derivations (ComputedValue, Reaction) with a deterministic update scheduler, and batching via startBatch/endBatch. React integration leverages hooks like useObserver and a UniversalFinalizationRegistry for reaction cleanup. The library has extensive test coverage (unit, integration, performance benchmarks) and is actively maintained with a clear deprecation path. Notable technical strengths include the lazy evaluation of computed values, the efficient binding algorithm for dependency tracking, and the decoupled annotation system that supports both decorator and makeObservable styles.
Machina.js is a TypeScript-first finite state machine library that provides both single-client and multi-client FSM implementations. It features hierarchical state machines with event bubbling, deferred input processing, lifecycle hooks, and a built-in event system. The library includes static analysis tools for detecting unreachable states and transition loops, testing utilities with custom matchers, React integration hooks, and an ESLint plugin for FSM validation. Built as a monorepo with pnpm and turbo, it emphasizes type safety and compile-time validation of state configurations.