React: What problems does Redux solve?

Redux is a library for managing global state of the entire application.

Simple analogy: Imagine you are managing a hotel.

• Without Redux (only useState/Context): each department (reception, maids, restaurant) keeps their own records. When a guest asks "Order dinner to the room", reception calls the restaurant, the restaurant calls the maids... Information is lost, everyone is in chaos.
• With Redux: everyone has one common log (store). Reception recorded the order → restaurant saw it → maids saw it. Everyone knows what is happening.

Problems Redux solves:

1. Unpredictable state — when many components change shared data, it is hard to understand who changed what and when. Redux makes changes explicit through actions.
2. Implicit changes — in React state you can change directly (mutate). This leads to bugs. Redux requires returning a new object every time (immutability).
3. Debugging complexity — Redux DevTools allows: time travel (undo/redo actions), see all state changes and who triggered them.
4. Prop-drilling — when you need to pass data through 10 levels of components. Context solves this, but for frequent updates Context is slow.

How Redux works:

• Store — one common object with data (single source of truth).
• Action — description of what happened: { type: "ADD_TODO", payload: "Buy milk" }.
• Reducer — pure function: (old_state, action) → new_state.
• Dispatch — sending action to store: dispatch(addTodo("Buy milk")).

Redux Toolkit (modern Redux): Reduces code several times. With Immer you can "mutate" state, and Redux will create a copy itself.