Top 20 React Interview Questions and Answers

This article presents 20 essential React interview questions with detailed answers, covering a range of fundamental concepts to help you prepare effectively.

1. What is React?

Answer: React is a declarative, efficient, and flexible JavaScript library for building user interfaces (UIs) or UI components. It allows developers to create complex UIs from small and isolated pieces of code called “components.” React focuses on the view layer in MVC (Model-View-Controller) or MVVM (Model-View-ViewModel) architectures.

2. What are the main features of React?

Answer:

• JSX (JavaScript XML): A syntax extension that allows you to write HTML-like structures within JavaScript code.
• Component-Based Architecture: UIs are built as a composition of reusable and independent components.
• Virtual DOM: React uses an in-memory representation of the DOM, enabling efficient updates.
• Declarative Programming: You describe the desired UI for a given state, and React handles the DOM updates.
• Unidirectional Data Flow: Data typically flows down from parent to child components via props.
• Hooks: Introduced in React 16.8, allowing functional components to use state and other React features.

3. What is JSX? Why do we use it?

Answer: JSX (JavaScript XML) is a syntax extension to JavaScript that allows you to write HTML-like structures directly within your JavaScript code. It gets transformed into regular JavaScript function calls that create React elements. We use it because it provides a more intuitive and familiar syntax for developers accustomed to HTML, improves readability, and allows embedding JavaScript expressions within the markup.

4. What are components in React? What are the two main types?

Answer: Components are the building blocks of React applications. They are independent and reusable pieces of code that define a part of the UI. The two main types are:

• Functional Components: Simple JavaScript functions that accept props as arguments and return JSX. They can now manage state and lifecycle behavior with Hooks.
• Class Components: ES6 classes that extend React.Component. They can have their own state, lifecycle methods, and handle more complex logic.

5. What are props in React? How are they passed?

Answer: Props (short for “properties”) are read-only arguments passed from a parent component to its child components. They are a way for parent components to send data and instructions down the component tree. Props are passed by adding attributes to the child component’s JSX tag in the parent component’s render() method.

6. What is state in React? How is it managed?

Answer: State is data that is local to a component and can change over time. It represents the dynamic parts of the UI.

• Class Components: State is typically initialized in the constructor() using this.state = { ... } and updated using this.setState().
• Functional Components: State is managed using the useState Hook, which returns a state value and a function to update it.

7. What is the difference between props and state?

Answer:

• Data Flow: Props are passed down (unidirectional), while state is managed within a component.
• Mutability: Props are read-only from the child’s perspective, while state can be modified within the component.
• Control: The parent component controls the props it passes, while the component controls its own state.

8. What is the significance of the key prop in React lists?

Answer: The key prop is a special string attribute that should be included when creating lists of elements in React. It helps React identify which items in the list have changed, been added, or been removed. Using stable and unique keys allows React to efficiently update and re-render only the necessary elements, maintaining component state and improving performance.

9. What is the virtual DOM? How does React use it?

Answer: The virtual DOM (VDOM) is an in-memory representation of the actual DOM. React uses it to optimize UI updates. When state or props change, React creates a new virtual DOM tree and compares it to the previous one (the “diffing” process). It then calculates the minimal set of changes needed and updates only those specific parts in the actual DOM (the “patching” process), leading to better performance.

10. What are React Hooks? Why were they introduced?

Answer: React Hooks are functions that let you “hook into” React state and lifecycle features from functional components. They were introduced to:

• Make stateful logic reusable among components.
• Simplify complex components by breaking them into smaller functions.
• Allow using state and other React features without writing classes.

11. Explain the useState Hook.

Answer: The useState Hook is used to add state to functional components. It takes an initial value as an argument and returns an array containing two elements: the current state value and a function to update that value. Calling the update function triggers a re-render of the component.

const [count, setCount] = useState(0);

12. Explain the useEffect Hook.

Answer: The useEffect Hook is used to perform side effects in functional components. These can include data fetching, subscriptions, or manually changing the DOM. It takes a function and an optional dependency array as arguments. The effect function runs after every render (by default), but you can control when it runs by providing the dependency array.

useEffect(() => {
  // This runs after every render
  document.title = `You clicked ${count} times`;

  return () => {
    // This is the cleanup function, runs before the next effect or unmount
  };
}, [count]); // Only re-run the effect if 'count' changes

13. What is the purpose of the dependency array in useEffect?

Answer: The dependency array in useEffect controls when the effect function will re-run.

• Empty array []: The effect runs only once after the initial render and the cleanup function runs once before unmount.
• Array with values [count, otherProp]: The effect runs after the initial render and whenever any of the values in the dependency array change. The cleanup function runs before the next effect or unmount.
• No array: The effect runs after every render.

14. What is the useContext Hook? How does it help with prop drilling?

Answer: The useContext Hook allows functional components to subscribe to React context. Context provides a way to pass data through the component tree without having to pass props down manually at every level (prop drilling). You need to create a Context using React.createContext() and provide its value using a Context.Provider higher up in the component tree. Components can then consume this value using useContext(MyContext).

15. What is Redux? What are its core principles?

Answer: Redux is a predictable state management library for JavaScript applications. Its core principles are:

• Single Source of Truth: The entire application state is stored in a single store.
• State is Read-Only: The only way to change the state is to dispatch an action, which is a plain JavaScript object describing the change.
• Changes are Made with Pure Functions: Reducers are pure functions that specify how the state changes in response to an action.

16. Explain the Redux flow (Action, Reducer, Store, Dispatch).

Answer:

1. Action: A plain JavaScript object that describes an event that has occurred. It typically has a type property and may include other data (payload).
2. Dispatch: An action is sent to the Redux store using the dispatch() function.
3. Reducer: A pure function that takes the current state and an action as arguments and returns a new state based on the action type.
4. Store: Holds the application state. When an action is dispatched, the store passes the current state and the action to the reducer. The reducer returns a new state, which becomes the new state of the store, and any subscribed components are notified of the change.

17. What is React Router? What are its main components?

Answer: React Router is a popular library for declarative routing in React applications. Its main components include:

• <BrowserRouter> or <HashRouter>: Provide the routing context to their children. <BrowserRouter> uses the HTML5 history API for clean URLs, while <HashRouter> uses the hash portion of the URL.
• <Route>: Used to define a mapping between a specific URL path and a React component that should be rendered when that path matches.
• <Link> or <NavLink>: Used for declarative navigation. <Link> creates an <a> tag that prevents a full page reload, while <NavLink> is a special type of <Link> that can style itself when it’s active.
• Switch (in older versions) or Routes (in v6): Used to render the first route that matches the current location.
• useParams(), useLocation(), useHistory() (in older versions) or useNavigate() (in v6): Hooks that provide access to routing information and navigation functions within components.

18. How do you handle form submissions in React?

Answer: Form submissions in React are typically handled by attaching an event handler to the <form> element’s onSubmit event. Inside the handler, you can prevent the default form submission behavior using event.preventDefault(), access form data (either through controlled components using state or uncontrolled components using refs), perform validation, and then send the data to an API or process it as needed.

19. What are controlled components in React?

Answer: In React, a controlled component is a form element where the value of the input is controlled by React state. The component renders the current state value, and any changes to the input (e.g., user typing) trigger an event handler that updates the state. This, in turn, re-renders the component with the new value, making React the single source of truth for the form element’s data.

20. What are some common performance optimization techniques in React?

Answer: Common performance optimization techniques in React include:

• Using React.memo for functional components and shouldComponentUpdate for class components: To prevent unnecessary re-renders when props haven’t changed.
• Using useCallback and useMemo Hooks: To memoize functions and expensive computations, passing stable references to child components.
• Virtualization (e.g., using libraries like react-virtualized or react-window): For rendering large lists efficiently by only rendering the visible items.
• Code splitting using React.lazy and Suspense: To load components on demand, reducing the initial bundle size.
• Avoiding unnecessary state updates: Only updating state when truly necessary.
• Optimizing event handlers: Debouncing or throttling frequently triggered events.
• Using production builds: Ensuring the application is built in production mode to leverage optimizations.