Effector is a reactive state management library for JavaScript/TypeScript with a domain-driven design philosophy. It models application logic using events (triggered actions), stores (state holders), effects (side effect handlers), and watchers (reactive observers). Unlike Redux's action-reducer pattern, Effector uses a more functional, reactive approach inspired by domain-driven design principles. Effector is used in large-scale applications (especially in Russia and Europe, where it originated) where complex data flows and side effects require sophisticated management. It's smaller than Redux, more powerful than Zustand, and increasingly adopted in TypeScript-first projects.