What is the difference between props and state?

Props are read-only inputs passed from a parent; state is data managed inside a component that can change over time and trigger re-renders.

When does useEffect run?

After render. With no dependency array it runs after every render; with an empty array only once on mount; with dependencies when those values change. Return a function to clean up.

What are the rules of hooks?

Call hooks only at the top level of a component or custom hook, never inside loops, conditions, or nested functions, and only from React functions.

useState vs useReducer?

useState is simplest for independent values; useReducer suits complex state with multiple related transitions or when the next state depends on the previous.

Why do I need keys in lists?

Keys help React identify which items changed, were added, or removed, so it can update the DOM efficiently. Use stable, unique ids rather than array indexes when possible.

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Reference

React Cheat Sheet

Modern React with TypeScript — components, hooks, events, context and performance. Click any card to copy.