State Management Interview Questions

State is a mutable data store internal to a component that determines the component's behavior and rendering. Unlike props which are read-only and passed from parent to child, state can be modified using setState() in class components or state updater functions in hooks, triggering re-renders when changed.

useState is a React hook that adds state management to functional components. It returns an array with two elements: the current state value and a function to update it. The hook accepts an initial state value and can be used multiple times in a single component for different state variables.

In class components, state is updated using this.setState(). It can accept either an object with new state values or a function that receives previous state as an argument. setState() performs shallow merging and updates are asynchronous for performance optimization.

useState is simpler and good for managing independent pieces of state. useReducer is preferred for complex state logic, especially when state updates depend on multiple factors or when one action should update multiple state values. useReducer follows a Redux-like pattern with actions and reducers.

AsyncStorage is a simple, unencrypted, asynchronous, persistent, key-value storage system that is global to the app. It's commonly used for storing user preferences, tokens, and other data that needs to persist across app restarts.

Context API provides a way to pass data through the component tree without manually passing props. It's useful for sharing global data like themes, user authentication, or language preferences. Should be used when data needs to be accessible by many components at different nesting levels.

Form state can be handled using controlled components with useState, or through form management libraries like Formik or React Hook Form. State typically includes input values, validation errors, and form submission status.

useEffect handles side effects in functional components, including state-related operations like data fetching, subscriptions, or manual DOM manipulations. It runs after render and can clean up effects by returning a function.

State can be shared between components through: 1) Lifting state up to a common ancestor, 2) Using Context API, 3) Implementing state management libraries like Redux or MobX, 4) Using custom hooks for shared state logic.

Common pitfalls include: 1) Mutating state directly instead of using setState/useState, 2) Async state updates race conditions, 3) Over-using global state, 4) Not considering component re-renders, 5) Improper state structure leading to performance issues.

Redux implements a unidirectional data flow with a single store holding the application state. Actions describe state changes, reducers specify how actions transform state, and components connect to the store using hooks or HOCs. Middleware handles side effects.

Redux Toolkit is the official toolset for Redux development. It simplifies store setup, reduces boilerplate through createSlice, handles immutable updates with Immer, and includes RTK Query for data fetching. It provides utilities for common Redux patterns.

State persistence involves: 1) Using AsyncStorage or similar storage solutions, 2) Implementing redux-persist for Redux state, 3) Handling storage serialization/deserialization, 4) Managing storage versioning, 5) Implementing proper error handling and fallbacks.

MobX offers simpler setup than Redux, automatic tracking of state dependencies, and more flexibility in state structure. Drawbacks include potential overuse of observables, harder debugging due to automatic updates, and less predictable state changes.

Real-time updates can be handled through: 1) WebSocket connections with state synchronization, 2) Push notifications with state updates, 3) Polling with proper state merging, 4) Optimistic updates with rollback handling.

Custom hooks are JavaScript functions that use React hooks to encapsulate reusable state logic. They help abstract complex state management, share stateful logic between components, and improve code organization and reusability.

Form validation state involves: 1) Tracking input values and errors, 2) Implementing validation rules and error messages, 3) Managing submission state, 4) Handling async validation, 5) Coordinating multiple form fields' validation state.

Middleware intercepts actions before they reach reducers, enabling side effects, async operations, logging, and routing. Common middleware includes redux-thunk for async actions, redux-saga for complex workflows, and custom middleware for specific needs.

Context optimization includes: 1) Split contexts by functionality, 2) Memoize context values, 3) Use context selectors to prevent unnecessary rerenders, 4) Implement context providers at appropriate levels, 5) Consider alternative solutions for frequently changing values.

Complex state patterns include: 1) State machines with XState, 2) Command pattern for actions, 3) Observable pattern for state streams, 4) Reducer composition for modular state logic, 5) Event sourcing for state history.

Offline-first state management requires: 1) Local state persistence, 2) Conflict resolution strategies, 3) Queue management for offline actions, 4) State synchronization upon reconnection, 5) Optimistic UI updates with proper error handling.

Large-scale state management involves: 1) Domain-driven state organization, 2) State normalization, 3) Module-based state splitting, 4) Lazy loading state modules, 5) State hydration strategies, 6) Performance optimization patterns.

Multi-window state management requires: 1) State synchronization between windows, 2) Shared storage mechanisms, 3) Event-based communication, 4) Consistent state updates across windows, 5) Handle window lifecycle events.

Microservices state management involves: 1) Domain-based state separation, 2) Service boundary definition, 3) State consistency patterns, 4) Event-driven state updates, 5) Distributed state management strategies.

Real-time collaboration requires: 1) Operational transformation or CRDT implementation, 2) State conflict resolution, 3) Presence management, 4) State versioning, 5) Optimistic updates with proper conflict handling.

Complex async patterns include: 1) Saga patterns for orchestration, 2) State machines for workflow management, 3) Queue-based state updates, 4) Retry and recovery strategies, 5) Transaction-like state updates.

AR/VR state management involves: 1) Real-time state updates for position/orientation, 2) Scene graph state management, 3) Object interaction state, 4) Performance optimization for state updates, 5) Platform-specific state handling.