Top 25 React Interview Questions (And How to Actually Answer Them)

We run mock interviews every Saturday, and the same React questions come up batch after batch. Here are the ones worth preparing properly.

Fundamentals

1. What is the virtual DOM and why does React use it? Talk about reconciliation and batching, not just "it's faster".
2. Difference between state and props? Ownership and mutability.
3. What are keys and why do they matter in lists? Explain what goes wrong when you use the array index.
4. Controlled vs uncontrolled components.
5. What does `useEffect` actually do, and when does it run?

Hooks

1. Why can't you call hooks inside conditions?
2. Explain useMemo vs useCallback — and when not to use them.
3. How would you write a custom hook for fetching data?
4. What problem does useRef solve beyond DOM access?
5. What is stale closure and how does it show up with setInterval?

State management

1. When do you reach for Context vs a library like Zustand or Redux?
2. How do you avoid unnecessary re-renders with Context?
3. Explain lifting state up with an example.

Performance

1. What is code splitting and how do you do it in React?
2. How does React.memo work, and why might it make things slower?
3. How would you debug a component that re-renders too often?

Modern React

1. Server Components vs Client Components — what runs where?
2. What are Suspense boundaries?
3. How do you handle forms with Server Actions?

Practical / coding

1. Build a debounced search input.
2. Implement a simple modal with focus trapping.
3. Write a component that paginates an API.
4. Fix a given component that has a memory leak.

Behavioural

1. Walk us through a bug you found in production.
2. How do you review someone else's pull request?

How to prepare

Don't memorise answers. For every question, build a 20-line example in CodeSandbox. When you've written the stale-closure bug yourself, you never forget the explanation.