Redux Interview Questions — Easy

• Definition: A predictable state container for JavaScript apps. State lives in one store; the only way to change it is dispatching actions into pure reducers.
• Problems solved:
  • Prop drilling — any component can subscribe to exactly the slices it needs.
  • Unpredictable mutations — immutability makes every change traceable.
  • Scattered state logic — transitions are centralized and testable.
• Best suited to complex, shared client state; overkill for purely local UI state.

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• Single source of truth — the entire app state is one object tree stored in one store.
• State is read-only — components never mutate; they dispatch plain action objects describing what happened.
• Changes via pure functions — reducers take (previousState, action) and return the next state, with no side effects. These three rules give you time-travel debugging, deterministic tests and trivial undo/redo foundations.

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• Store — holds state, exposes getState(), dispatch(), subscribe().
• Action — a plain object { type, payload? } describing an event.
• Action creator — a function returning an action (keeps dispatch sites clean and typed).
• Reducer — pure function producing next state per action.
• Dispatch — the only mutation entry point.
• Selector — function reading derived data from state (state => state.cart.items).

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• An action is the payload object itself: { type: 'cart/add', payload: { id } }.
• An action creator is the function that builds it: const addToCart = id => ({ type: 'cart/add', payload: { id } }).
• Why bother with creators: centralizes action shapes, gives TypeScript a single typed factory, keeps components free of string literals, and (with thunks/sagas) becomes the place where async logic starts.
• With Redux Toolkit you rarely hand-write them — createSlice generates creators automatically from reducer names.

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function cartReducer(state = initialState, action) {
  switch (action.type) {
    case 'cart/add':
      return { ...state, items: [...state.items, action.payload] };
    default:
      return state;
  }
}

Rules:

1. Pure — same inputs ⇒ same output; no API calls, no Date.now/Math.random.
2. No mutation — spread/copy every touched level.
3. Default case returns state unchanged (enables cheap bailouts).
4. Never return undefined — React state reads will explode.

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