React JS is one of the most popular JavaScript libraries for building user interfaces, especially single-page applications. Interviewers for React JS Developer roles focus on your understanding of React concepts, component lifecycle, state management, hooks, and performance optimization. Below are 25 commonly asked React JS Developer interview questions with answers to help you prepare effectively.
Q1. What is React?
React is a JavaScript library developed by Facebook for building reusable UI components and creating fast, interactive web applications.
Q2. What are the key features of React?
Virtual DOM, JSX, components, one-way data binding, and declarative programming are key features.
Q3. What is JSX?
JSX is a syntax extension that allows you to write HTML-like code within JavaScript, which React then transforms into React elements.
Q4. What are components in React?
Components are reusable, independent pieces of UI. They can be class-based or functional.
Q5. What is the difference between a class component and a functional component?
Class components can have state and lifecycle methods, while functional components are simpler and can use hooks to manage state and side effects.
Q6. What are props in React?
Props are inputs passed to components to customize them. They are read-only and cannot be modified by the component receiving them.
Q7. What is state in React?
State is a local data storage that can be changed by the component and affects what is rendered.
Q8. What are hooks in React?
Hooks are functions like useState and useEffect that let functional components manage state and lifecycle events.
Q9. Explain the useState hook.
useState is a hook that lets you add state to functional components by returning a state variable and a function to update it.
Q10. What is the useEffect hook?
useEffect lets you perform side effects like data fetching or DOM manipulation in functional components.
Q11. What is the Virtual DOM?
The Virtual DOM is a lightweight copy of the real DOM that React uses to optimize updates by only changing what is necessary.
Q12. How does React handle events?
React uses synthetic events that wrap browser native events for cross-browser compatibility.
Q13. What is the difference between controlled and uncontrolled components?
Controlled components have their input values controlled by React state, whereas uncontrolled components maintain their own internal state.
Q14. How do you optimize React application performance?
Use techniques like memoization, code-splitting, lazy loading, avoiding unnecessary re-renders, and using React.PureComponent or React.memo.
Q15. What is React Router?
React Router is a library that enables navigation and routing in single-page React applications.
Q16. What is Redux?
Redux is a state management library often used with React to manage global application state in a predictable way.
Q17. How do you pass data between React components?
Data is passed from parent to child components via props. For sibling or distant components, state management libraries or context API can be used.
Q18. What is Context API?
Context API allows sharing state globally across components without prop drilling.
Q19. What are React fragments?
Fragments let you group multiple elements without adding extra nodes to the DOM.
Q20. How do you handle forms in React?
By using controlled components, handling input events, and managing form submission via React state and event handlers.
Q21. What is the significance of keys in React lists?
Keys help React identify which items have changed, are added, or removed, improving rendering efficiency.
Q22. How does React update the DOM?
React uses reconciliation by comparing the Virtual DOM with the real DOM and updating only the parts that changed.
Q23. What is error boundary in React?
Error boundaries are components that catch JavaScript errors anywhere in their child component tree and display a fallback UI.
Q24. How do you test React components?
Using testing libraries like Jest and React Testing Library to perform unit and integration testing.
Q25. What are Higher-Order Components (HOC)?
HOCs are functions that take a component and return a new component with enhanced features or logic.