React is currently the most popular frontend library in the world. Whether you are a fresher looking for your first job or an experienced developer switching stacks, you are guaranteed to face React questions in modern frontend interviews. Below, we break down the most critical concepts you must understand, complete with examples and "Pro Tips" to help you stand out.

1 What is the Virtual DOM and how does it work?

The Virtual DOM (VDOM) is an in-memory representation of the Real DOM. Manipulating the Real DOM is extremely slow and expensive. When a component's state changes, React updates the Virtual DOM first.

It then compares the updated Virtual DOM with a snapshot of the previous Virtual DOM (a process called Diffing). Once it identifies exactly what changed, it updates only those specific nodes in the Real DOM (a process called Reconciliation).

Pro Tip: Don't just say "it makes React fast." Explain the Diffing algorithm (which operates in O(n) time) and mention Reconciliation.
2 Explain the difference between functional and class components.

Historically, Class components were required if you needed to manage state or use lifecycle methods (like componentDidMount). Functional components were simply "dumb" components that accepted props and returned JSX.

With the introduction of React Hooks in version 16.8, functional components can now manage state and side effects. Today, functional components are the industry standard because they are less verbose, easier to test, and avoid the confusing behavior of the this keyword in JavaScript classes.

3 What are React Hooks? Name a few common ones.

Hooks are functions that let you "hook into" React state and lifecycle features from functional components.

  • useState: Allows you to add state variables to functional components.
  • useEffect: Lets you perform side effects (like data fetching, subscriptions, or manually changing the DOM). It serves the same purpose as componentDidMount, componentDidUpdate, and componentWillUnmount in React classes.
  • useContext: Allows you to subscribe to React context without introducing nesting.
  • useMemo / useCallback: Used for performance optimization by memoizing values or functions.
// Example of useState const [count, setCount] = useState(0);
💡
Keep Practicing!
Understanding the theory is only half the battle. Make sure you build small projects using these concepts so you can discuss them practically during your interview.