Advanced React Interview Questions and Answers
By admin | 9 months ago
1. How does React's reconciliation algorithm work?
Answer: React's reconciliation algorithm, also known as the diffing algorithm, compares the current tree with the previous one to identify what has changed. It updates the DOM as efficiently as possible by re-rendering only the components that have changed state or props, rather than the entire DOM tree.
2. Explain the concept of higher-order components (HOC).
Answer: A higher-order component (HOC) is a function that takes a component and returns a new component, allowing you to reuse component logic. It’s a pattern that emerges from React’s compositional nature.
function withLogging(WrappedComponent) { return class extends React.Component { componentDidMount() { console.log(`Component ${WrappedComponent.name} mounted`); } render() { return <WrappedComponent {...this.props} />; } }; }
3. What are render props?
Answer: Render props refer to a technique for sharing code between React components using a prop whose value is a function.
<DataProvider render={data => ( <h1>{`Hello, ${data.target}`}</h1> )} />
4. How do you handle errors in React components?
Answer: Error boundaries are React components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of the component tree that crashed.
class ErrorBoundary extends React.Component { constructor(props) { super(props); this.state = { hasError: false }; } static getDerivedStateFromError(error) { return { hasError: true }; } render() { if (this.state.hasError) { return <h1>Something went wrong.</h1>; } return this.props.children; } }
5. Explain React's Context API and when you would use it.
Answer: The Context API allows you to share values (contexts) across the component tree without having to explicitly pass props through every level. It's useful for sharing data that can be considered “global” for a tree of React components, such as the current authenticated user, theme, or preferred language.
6. Describe the use of refs in React.
Answer: Refs provide a way to access DOM nodes or React elements created in the render method. They can be used for managing focus, text selection, or media playback.
class MyComponent extends React.Component { constructor(props) { super(props); this.myRef = React.createRef(); } render() { return <div ref={this.myRef} />; } }
7. What is the purpose of the key prop in a list?
Answer: The `key` prop is a unique identifier that helps React identify which items have changed, are added, or are removed from lists. It aids in efficient updates of the user interface.
8. Explain how to implement code-splitting in React.
Answer: Code-splitting can be implemented using dynamic `import()` syntax which allows you to split your code into separate chunks and load them on demand.
import React, { Suspense } from 'react'; const OtherComponent = React.lazy(() => import('./OtherComponent')); function MyComponent() { return ( <Suspense fallback={<div>Loading...</div>}> <OtherComponent /> </Suspense> ); }
9. How does React use the Virtual DOM to improve performance?
Answer: React uses the Virtual DOM to optimize rendering. It's a lightweight copy of the actual DOM. React compares the Virtual DOM with a snapshot taken right before the update (using the diffing algorithm) to identify the minimal changes required, then updates only those elements in the real DOM.
10. What is the use of the useCallback hook?
Answer: The `useCallback` hook returns a memoized version of the callback function that only changes if one of the dependencies has changed. It’s useful for preventing unnecessary re-renders in components that rely on reference equality to avoid unnecessary updates.
const memoizedCallback = useCallback( () => { doSomething(a, b); }, [a, b], );
11. Explain the use of the useMemo hook.
Answer: The `useMemo` hook returns a memoized value. It only recalculates the memoized value when one of the dependencies has changed. This optimization helps to avoid expensive calculations on every render.
const memoizedValue = useMemo(() => computeExpensiveValue(a, b), [a, b]);
12. How can you implement global state management in React without external libraries?
Answer: Global state management can be implemented using React's Context API and useReducer hook to create a store-like solution for state management without the need for external libraries like Redux.
const StoreContext = React.createContext(); const StoreProvider = ({ children }) => { const [state, dispatch] = useReducer(reducer, initialState); return ( <StoreContext.Provider value={{ state, dispatch }}> {children} </StoreContext.Provider> ); }; ## 13. What are Fragments and why are they used? **Answer:** Fragments let you group a list of children without adding extra nodes to the DOM. This is useful when rendering components that require multiple elements, but you don’t want to add an extra element (like a `<div>`) just to satisfy React’s single root element requirement. ```jsx <React.Fragment> <ChildA /> <ChildB /> <ChildC /> </React.Fragment>
14. Explain the concept of Prop Drilling and how to avoid it.
Answer: Prop Drilling is the process by which you pass data from part of a React component tree to another part by going through other components that do not need the data. It becomes cumbersome for deeply nested components. To avoid prop drilling, you can use React's Context API or state management libraries like Redux to share state across the components more efficiently.
15. What is Redux and how does it relate to React?
Answer: Redux is a predictable state container for JavaScript apps. It helps you write applications that behave consistently, run in different environments (client, server, and native), and are easy to test. While Redux can be used with any view library, it's most commonly used with React. It provides a central store that holds all state for your application, with rules ensuring that state can only be updated in a predictable fashion.
16. Explain how to implement code-splitting in your React app.
Answer: Code-splitting is a technique to split your code into small chunks which you then load on demand. In React, you can use dynamic `import()` syntax in conjunction with React.lazy and `
const OtherComponent = React.lazy(() => import('./OtherComponent')); function MyComponent() { return ( <div> <React.Suspense fallback={<div>Loading...</div>}> <OtherComponent /> </React.Suspense> </div> ); }
17. How do you create Refs in React and what are they used for?
Answer: Refs provide a way to access DOM nodes or React elements created in the render method. In React, you can create refs using `React.createRef()` and attach them to elements via the `ref` attribute. Refs are used for managing focus, text selection, or media playback, triggering imperative animations, or integrating with third-party DOM libraries.
class MyComponent extends React.Component { constructor(props) { super(props); this.myRef = React.createRef(); } render() { return <div ref={this.myRef} />; } }
18. What are the rules for custom Hooks?
Answer: Custom Hooks are JavaScript functions whose name starts with ”use” and that may call other Hooks. The rules for custom Hooks are:
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Only call Hooks at the top level (not inside loops, conditions, or nested functions).
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Only call Hooks from React function components or other custom Hooks.
19. Explain how Context and Redux differ in state management.
Answer: Both Context and Redux provide ways to pass data through the component tree without having to pass props down manually at every level. Context is great for passing down data that can be considered “global” for a tree of React components, such as current authenticated user, theme, or preferred language. Redux, on the other hand, is more suited for managing larger state that needs to be accessed across many components, provides a more structured way to manage data flow and state, and offers powerful middleware capabilities.
20. Discuss the importance of keys in React lists.
Answer: Keys help React identify which items have changed, are added, or are removed in lists. They should be given to the elements inside the array to give the elements a stable identity. Using keys improves the performance of component updates, especially for dynamic lists, by allowing React to reuse existing elements.
21. How do you optimize React application performance?
Answer: To optimize React application performance, consider the following strategies:
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Use the `React.memo` for memoizing components.
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Implement code splitting using `React.lazy` and
Suspense. -
Utilize `shouldComponentUpdate
,PureComponent, orReact.memo` to avoid unnecessary re-renders. -
Use `useCallback` and `useMemo` hooks to memoize functions and values.
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Keep component state local where necessary.
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Use virtualization for large lists and tables.
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Analyze and optimize bundle size.
22. Explain the use of `useCallback` and `useMemo` hooks.
Answer: `useCallback` returns a memoized callback function. This is
useful when passing callbacks to optimized child components that rely on reference equality to prevent unnecessary renders. `useMemo` returns a memoized value. It’s used to compute expensive values only when the dependencies have changed, thus optimizing performance by avoiding expensive calculations on every render.
const memoizedCallback = useCallback( () => { doSomething(a, b); }, [a, b], ); const memoizedValue = useMemo(() => computeExpensiveValue(a, b), [a, b]); feel free to checkout latest [react jobs in kerala](https://keralait.dev/?q=react)