Redux is a predictable state container for JavaScript applications, most commonly used with React but framework-agnostic. It centralizes application state into a single immutable store, enforces unidirectional data flow, and enables powerful debugging capabilities like time-travel and hot reloading. Redux follows three core principles: single source of truth, state is read-only, and changes are made with pure functions (reducers). While modern alternatives exist (Zustand, Recoil, Jotai), Redux remains widely adopted in large-scale applications where predictability, middleware extensibility, and DevTools integration are critical. Redux Toolkit (RTK) is now the official, opinionated way to write Redux — simplifying boilerplate with utilities like createSlice, configureStore, and createAsyncThunk, while including Immer for safe "mutative" updates and RTK Query for data fetching.

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