React.js interview questions

Beginner Level

1. What is React?

o Answer: React is a JavaScript library for building user interfaces, particularly single-page applications where you can create reusable UI components. It allows developers to create large web applications that can change data, without reloading the page.

2. What are components in React?

o Answer: Components are the building blocks of a React application. They are reusable pieces of UI that can be defined as either functional or class components. Functional components are simple JavaScript functions that return JSX, while class components are ES6 classes that extend from React.Component.

3. What is JSX?

o Answer: JSX (JavaScript XML) is a syntax extension for JavaScript that looks similar to XML or HTML. It allows developers to write HTML-like syntax in their JavaScript code, which gets compiled to React.createElement() calls. JSX makes it easier to visualize the UI structure.

4. What are props in React?

o Answer: Props (short for properties) are a mechanism for passing data and event handlers from a parent component to a child component. They are read-only and help maintain a unidirectional data flow in React applications.

5. What is state in React?

o Answer: State is an object that holds data that may change over the lifecycle of a component. Unlike props, state is managed within the component, and updates to the state can trigger a re-render of the component.

6. How do you handle events in React?

o Answer: Events in React can be handled using camelCase syntax. You can attach an event handler to a JSX element using curly braces. For example:

<button onClick={this.handleClick}>Click me</button>

7. What is the virtual DOM?

o Answer: The virtual DOM is an in-memory representation of the real DOM elements. React creates a virtual DOM to optimize updates. When the state of an object changes, React first changes the virtual DOM, then compares it with the previous version and finally updates only the parts of the real DOM that have changed, improving performance.

8. What is the purpose of useEffect?

o Answer: The useEffect hook allows you to perform side effects in functional components, such as data fetching, subscriptions, or manually changing the DOM. It runs after the first render and after every update, but you can control when it runs by passing a dependency array.

Intermediate Level

1. What are hooks in React?

o Answer: Hooks are functions that let you use state and other React features in functional components. The most common hooks include useState, useEffect, and useContext. They enable functional components to have state and lifecycle methods without converting them into class components.

2. How do you manage state in a React application?

o Answer: State can be managed using the built-in useState hook for local component state, or through more advanced solutions like Context API for global state management, and libraries like Redux or MobX for more complex state management scenarios.

3. What is the difference between controlled and uncontrolled components?

o Answer: Controlled components are those where React controls the form data through the component's state, meaning the form elements (like input fields) get their values from the state. Uncontrolled components, on the other hand, maintain their own internal state and can be accessed through refs. Controlled components provide more control and validation, while uncontrolled components are often easier to integrate with non-React code.

4. How do you pass data from a parent component to a child component?

o Answer: Data can be passed from a parent to a child component using props. The parent component can define a prop and pass it to the child component as follows:

5. What are higher-order components (HOCs)?

o Answer: HOCs are functions that take a component and return a new component. They are used for reusing component logic. A common example is a HOC that adds logging or authentication to a component:

function withLogging(WrappedComponent) {

return function(props) {

console.log('Rendering', WrappedComponent.name);

return <WrappedComponent {...props} />;

};

}

6. What is the purpose of useContext?

o Answer: The useContext hook allows you to access context values directly within functional components. It provides a way to pass data through the component tree without having to pass props down manually at every level. For example:

const value = useContext(MyContext);

7. How can you optimize performance in a React application?

o Answer: Performance can be optimized by:

§ Using React.memo to prevent unnecessary re-renders of functional components.

§ Implementing code splitting with dynamic imports and React.lazy.

§ Avoiding inline functions in render methods.

§ Using useCallback and useMemo hooks to memoize values and functions.

§ Profiling components with React’s built-in Profiler.

8. What are React Fragments?

o Answer: Fragments allow you to group multiple elements without adding an extra node to the DOM. They can be declared using <React.Fragment> or shorthand <>...</>. For example:

return (

<React.Fragment>

</React.Fragment>

);

Advanced Level

1. What is the difference between React.memo and PureComponent?

o Answer: React.memo is a higher-order component that memoizes functional components, preventing them from re-rendering when their props do not change. PureComponent is a class component that implements a shallow comparison of props and state. Both can help optimize performance by preventing unnecessary re-renders.

2. What is the significance of keys in React?

o Answer: Keys are unique identifiers for elements in a list. They help React identify which items have changed, are added, or are removed, allowing for efficient updates. Using keys helps prevent performance issues and bugs in UI rendering.

3. How does React handle forms?

o Answer: React handles forms through controlled components, where form data is controlled by the state of the component. You typically handle changes with an onChange event handler, updating the state to reflect user input. Validation and submission can also be handled through event handlers.

4. What is the Context API, and how is it used?

o Answer: The Context API allows for global state management by creating a context that can be shared across components without prop drilling. You create a context with React.createContext(), then use Context.Provider to pass values and Context.Consumer or the useContext hook to access them in child components.

5. What are render props?

o Answer: Render props are a technique for sharing code between components using a prop that is a function. The function returns a React element, allowing for flexible rendering based on the props provided. For example:

const DataProvider = ({ render }) => {

const data = fetchData();

return render(data);

};

6. How can you test React components?

o Answer: React components can be tested using libraries like Jest and React Testing Library. You can write unit tests to check the rendering and behavior of components, simulate user interactions, and assert the expected output.

7. What is the purpose of useReducer?

o Answer: The useReducer hook is an alternative to useState for managing complex state logic in functional components. It is particularly useful when the next state depends on the previous one, or when managing multiple state values. It uses a reducer function to determine the new state:

const [state, dispatch] = useReducer(reducer, initialState);

8. How does server-side rendering (SSR) work with React?

o Answer: Server-side rendering is the process of rendering a React application on the server instead of the client. The server generates the HTML for the initial page load, which can improve performance and SEO. Frameworks like Next.js facilitate SSR with React by providing built-in routing and data fetching methods.

9. What are the differences between React Router v5 and v6?

o Answer: React Router v6 introduces a simplified API and improved features such as:

§ A new Routes component that replaces Switch.

§ Route nesting with simpler syntax.

§ Automatic route ranking.

§ The ability to use useRoutes for programmatic routing.

10. Explain the concept of suspense and lazy loading in React.

o Answer: Suspense allows components to "wait" for something before rendering, such as data fetching or code splitting. You can wrap parts of your application in a <Suspense> component to show a fallback UI while loading. Lazy loading, achieved with React.lazy, enables components to be loaded only when they are needed, improving the initial loading time of the application.

1. What is the difference between class components and functional components in React?

o Answer: Class components are ES6 classes that extend React.Component and can hold their own state and lifecycle methods. Functional components are simpler and are JavaScript functions that return JSX. With the introduction of hooks, functional components can also manage state and side effects, making them more powerful and preferred in modern React development.

2. What are lifecycle methods in React?

o Answer: Lifecycle methods are special methods that automatically get called at different stages of a component’s life in a class component. Common lifecycle methods include:

§ componentDidMount: Invoked immediately after a component is mounted.

§ componentDidUpdate: Invoked immediately after updating occurs.

§ componentWillUnmount: Invoked immediately before a component is unmounted and destroyed.

3. What is setState in React?

o Answer: setState is a method used to update the state in class components. It takes an object or a function as an argument and merges it with the current state. Calling setState triggers a re-render of the component to reflect the updated state.

4. How do you create a React application?

o Answer: You can create a new React application using Create React App (CRA) by running the following command in your terminal:

npx create-react-app my-app

This sets up a new React project with a preconfigured development environment.

5. What is the purpose of the key prop in React lists?

o Answer: The key prop helps React identify which items in a list have changed, been added, or removed. It should be a unique identifier for each item to ensure that the component’s state and structure are preserved during re-renders.

6. What is the default export in a React component?

o Answer: The default export allows a module to export a single value or object, which can be imported without curly braces. In a React component, you might have:

const MyComponent = () => {

return <div>Hello World</div>;

};

export default MyComponent;

7. How do you update the state based on the previous state in React?

o Answer: You can update the state based on the previous state by passing a function to setState, which receives the previous state as an argument:

this.setState((prevState) => ({ count: prevState.count + 1 }));

8. What is the use of props.children?

o Answer: props.children is a special prop that allows you to pass child components or elements into a component. It enables composition, allowing a component to render any nested elements:

const Container = (props) => {

return <div className="container">{props.children}</div>;

};

9. What is a higher-order function?

o Answer: A higher-order function is a function that takes another function as an argument or returns a function. In React, higher-order components (HOCs) are a common pattern that use higher-order functions to enhance or modify components.

10. What is the difference between == and === in JavaScript?

o Answer: == is the equality operator that checks for value equality after performing type coercion if necessary. === is the strict equality operator that checks for both value and type equality without coercion. It is generally recommended to use === to avoid unexpected results due to type coercion.

11. How do you conditionally render components in React?

o Answer: You can conditionally render components using JavaScript expressions, such as ternary operators or logical &&:

return (

<div>

{isLoggedIn ? <UserGreeting /> : <GuestGreeting />}

{showWarning && <WarningMessage />}

</div>

);

12. What is a React Hook?

o Answer: Hooks are functions that allow you to use state and other React features in functional components. Common hooks include useState, useEffect, and useContext. They enable functional components to have capabilities that were previously only available in class components.

13. What is the purpose of the render method in a class component?

o Answer: The render method is required in class components and is responsible for returning the JSX that defines what the component should display on the screen. It can also contain logic for conditional rendering or mapping through lists.

14. How do you bind methods in class components?

o Answer: In class components, methods can be bound to the component instance using the constructor or by using arrow functions. Here’s an example of binding in the constructor:

constructor(props) {

super(props);

this.handleClick = this.handleClick.bind(this);

}

15. What is the difference between useEffect with and without a dependency array?

o Answer: When you use useEffect without a dependency array, the effect runs after every render of the component. If you provide an empty dependency array ([]), the effect runs only once after the initial render, similar to componentDidMount. When you include specific dependencies, the effect runs only when those dependencies change.

16. What is JSX?

o Answer: JSX (JavaScript XML) is a syntax extension for JavaScript that looks similar to HTML. It allows you to write HTML-like code within JavaScript, which makes it easier to create React components. JSX is transpired to JavaScript using tools like Babel. For example:

const element = <h1>Hello, world!</h1>;

17. How do you pass data from a parent component to a child component?

o Answer: You can pass data from a parent component to a child component using props. The parent component specifies the props when rendering the child component:

const Parent = () => {

return <Child message="Hello from Parent" />;

};

const Child = (props) => {

return <p>{props.message}</p>;

};

18. What are controlled components in React?

o Answer: Controlled components are form elements whose value is controlled by React state. In controlled components, the form data is handled by the component's state, and any changes to the input are communicated through event handlers. For example:

const MyForm = () => {

const [value, setValue] = useState("");

const handleChange = (e) => {

setValue(e.target.value);

};

return <input type="text" value={value} onChange={handleChange} />;

};

19. What are uncontrolled components in React?

o Answer: Uncontrolled components are form elements that store their own state internally, and their values are not controlled by React. You can access their values using refs. For example:

const MyForm = () => {

const inputRef = useRef(null);

const handleSubmit = () => {

alert(inputRef.current.value);

};

return (

<div>

<button onClick={handleSubmit}>Submit</button>

</div>

);

};

20. What is the purpose of the defaultProps in React?

o Answer: defaultProps is a property that allows you to define default values for props in a component. If a prop is not provided by the parent component, the default value will be used. For example:

const MyComponent = ({ name }) => {

return <h1>Hello, {name}!</h1>;

};

MyComponent.defaultProps = {

name: "Guest",

};

21. What is the useState hook?

o Answer: The useState hook is a built-in hook that allows you to add state to functional components. It returns an array containing the current state and a function to update it. You can initialize the state by passing an initial value. For example:

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

22. What is the useEffect hook?

o Answer: The useEffect hook allows you to perform side effects in functional components, such as fetching data or directly interacting with the DOM. It runs after every render by default but can be controlled by providing a dependency array. For example:

useEffect(() => {

// Code to run on component mount or update

console.log("Component mounted or updated");

}, [dependency]);

23. What are error boundaries in React?

o Answer: Error boundaries are components that catch JavaScript errors in their child components, log those errors, and display a fallback UI instead of crashing the entire application. They are implemented using the componentDidCatch lifecycle method or the static getDerivedStateFromError method in class components.

24. What is prop drilling?

o Answer: Prop drilling refers to the process of passing data from a parent component down to deeply nested child components through props. This can lead to a cumbersome and hard-to-manage component structure. To avoid prop drilling, you can use state management libraries like Redux or React Context.

25. What is the purpose of the useContext hook?

o Answer: The useContext hook allows you to access context values in functional components. It simplifies consuming context by eliminating the need to wrap components in a Context.Consumer. For example:

const value = useContext(MyContext);

26. What is the difference between a controlled and uncontrolled component?

o Answer: In controlled components, form data is managed by React state, and any changes to the input are handled by event handlers. In uncontrolled components, form data is managed by the DOM, and you can access values using refs. Controlled components offer more control over form data, while uncontrolled components can be simpler for certain use cases.

27. What is the purpose of React.StrictMode?

o Answer: React.StrictMode is a tool for highlighting potential problems in an application. It activates additional checks and warnings for its descendants. It helps identify components with unsafe lifecycle methods, legacy API usage, and other issues during development without affecting the production build.

28. How can you prevent a component from re-rendering?

o Answer: You can prevent unnecessary re-renders by using React.memo for functional components or shouldComponentUpdate in class components. React.memo compares the props of the component and only re-renders if the props have changed:

const MyComponent = React.memo(({ value }) => {

return <div>{value}</div>;

});

29. What is the significance of key in React?

o Answer: The key prop is used to uniquely identify elements in a list of components. It helps React optimize rendering by keeping track of items and only updating the elements that have changed. Keys should be unique among siblings but do not need to be globally unique.

30. What are fragments in React?

o Answer: Fragments are a way to group multiple elements without adding extra nodes to the DOM. They allow you to return multiple elements from a component without needing a wrapper element. You can use <Fragment> or the shorthand <>...</> syntax:

return (

<h1>Hello</h1>

<h2>World</h2>

</>

);

31. What are hooks in React?

o Answer: Hooks are special functions that let you "hook into" React features like state and lifecycle methods in functional components. They were introduced in React 16.8 to allow functional components to manage state and side effects without needing to convert them into class components. Common hooks include useState, useEffect, and useContext.

32. How do you handle events in React?

o Answer: You handle events in React using camelCase syntax. Event handlers are passed as props to React components. For example:

const handleClick = () => {

alert("Button clicked!");

};

return <button onClick={handleClick}>Click Me</button>;

33. What is the useRef hook used for?

o Answer: The useRef hook is used to create a mutable reference that persists for the full lifetime of the component. It can be used to access a DOM element directly or to store any mutable value that doesn’t trigger a re-render when updated. For example:

const inputRef = useRef();

const focusInput = () => {

inputRef.current.focus();

};

return (

<button onClick={focusInput}>Focus the input</button>

</>

);

34. What is the purpose of the useMemo and useCallback hooks?

o Answer:

§ useMemo is used to optimize performance by memoizing expensive calculations and returning a cached result unless the dependencies change.

§ useCallback is used to memoize functions so that they are only recreated when their dependencies change, preventing unnecessary re-renders of child components. For example:

const memoizedValue = useMemo(() => computeExpensiveValue(a, b), [a, b]);

const memoizedCallback = useCallback(() => { /* function logic */ }, [dependency]);

35. What is the difference between null and undefined in JavaScript?

o Answer: null is an intentional absence of any object value and is a primitive value, whereas undefined indicates that a variable has been declared but has not yet been assigned a value. null is often used to signify "no value," while undefined signifies that something is missing or not initialized.

36. How do you create a form in React?

o Answer: You create a form in React using controlled components. You manage the input values using React state and handle form submission through event handlers. Here’s an example:

const MyForm = () => {

const [name, setName] = useState("");

const handleSubmit = (e) => {

e.preventDefault();

alert(`A name was submitted: ${name}`);

};

return (

<input type="text" value={name} onChange={(e) => setName(e.target.value)} />

<button type="submit">Submit</button>

</form>

);

};

37. What is a conditional rendering in React?

o Answer: Conditional rendering is a technique in React that allows you to render different components or elements based on certain conditions. This can be done using JavaScript expressions such as if statements, ternary operators, or logical AND (&&) operators. For example:

const Greeting = ({ isLoggedIn }) => {

return isLoggedIn ? <h1>Welcome Back!</h1> : <h1>Please Sign In</h1>;

};

38. How do you perform a fetch request in a React component?

o Answer: You can perform fetch requests in React using the fetch API inside the useEffect hook. This allows you to fetch data when the component mounts. Here's an example:

const App = () => {

const [data, setData] = useState([]);

useEffect(() => {

fetch("https://api.example.com/data")

.then((response) => response.json())

.then((data) => setData(data));

}, []);

return (

<div>

{data.map((item) => (

))}

</div>

);

};

39. What is the difference between functional components and class components?

o Answer: Functional components are simpler, stateless components that are defined as functions. They can use hooks to manage state and lifecycle methods. Class components are more complex and are defined as ES6 classes, with their own state and lifecycle methods. Functional components are preferred in modern React development for their simplicity and ability to leverage hooks.

40. What are the various ways to style React components?

o Answer: There are several ways to style React components, including:

§ CSS stylesheets: Traditional CSS files linked in the component.

§ Inline styles: Styles defined directly on elements using the style attribute.

§ CSS Modules: Scoped CSS files that avoid class name collisions.

§ Styled Components: A library for writing component-level styles in JavaScript.

§ Sass or Less: CSS preprocessors that offer more features like variables and nesting.

41. How can you manage global state in a React application?

o Answer: Global state in a React application can be managed using Context API or external state management libraries such as Redux, MobX, or Zustand. The Context API allows you to create a global store and provide it to components without prop drilling.

42. What is the purpose of the React.StrictMode wrapper?

o Answer: React.StrictMode is a tool for highlighting potential problems in an application. It helps identify components that may have unsafe lifecycle methods, legacy API usage, and other issues during development. It activates additional checks and warnings for its descendants but does not render any visible UI itself.

43. How do you handle form validation in React?

o Answer: You can handle form validation in React by checking the input values in the state when the form is submitted or during input changes. You can implement validation logic and display error messages based on the results. Libraries like Formik or React Hook Form can simplify this process as well.

44. What is the Fragment component used for?

o Answer: The Fragment component allows you to group multiple children without adding extra nodes to the DOM. It is often used to return multiple elements from a component's render method without wrapping them in a div. You can use it like this:

return (

<h1>Title</h1>

<p>Description</p>

</Fragment>

);

45. What are synthetic events in React?

o Answer: Synthetic events are a cross-browser wrapper around the native event in React. They are normalized so that you can handle events consistently across different browsers. Synthetic events have the same interface as native events and are used to handle events in a React application.

46. What is the purpose of the useLayoutEffect hook?

o Answer: The useLayoutEffect hook is similar to useEffect, but it fires synchronously after all DOM mutations. It is useful for reading layout from the DOM and synchronously re-rendering. You should use it sparingly, as it can cause performance issues if overused.

47. How do you handle multiple state variables in a functional component?

o Answer: You can manage multiple state variables using the useState hook for each variable. Alternatively, you can use a single state variable that holds an object containing all state values. For example:

const [state, setState] = useState({ name: '', age: 0 });

const handleChange = (e) => {

setState({ ...state, [e.target.name]: e.target.value });

};

48. What is the role of componentDidCatch in error handling?

o Answer: componentDidCatch is a lifecycle method in class components that is used to catch JavaScript errors in its child components, log those errors, and display a fallback UI. This helps prevent the entire application from crashing due to an error in a single component.

49. What is the significance of the render method in class components?

o Answer: The render method is a required method in class components that returns the JSX to be rendered. It can also contain logic for conditional rendering, mapping through arrays, and more. It is called during the initial mount and every time the component updates.

50. How do you manage side effects in React?

o Answer: You manage side effects in React using the useEffect hook in functional components. It allows you to perform tasks such as data fetching, subscriptions, or manually changing the DOM after rendering. You can specify dependencies to control when the effect runs.

General React Questions

1. What is React?

2. What are the main features of React?

3. What is the difference between React and other JavaScript frameworks?

4. What is a React component?

5. How do you create a functional component in React?

6. What is a class component in React?

7. What is JSX?

8. Why is JSX used in React?

9. What are props in React?

10. How do you pass props to a component?

Component Lifecycle

11. What is the component lifecycle in React?

12. What are the different lifecycle methods in class components?

13. What is the purpose of componentDidMount?

14. What is componentWillUnmount used for?

15. What is the difference between componentDidUpdate and componentWillUpdate?

State Management

16. What is state in React?

17. How do you set state in a class component?

18. What is the setState method?

19. How do you initialize state in a functional component?

20. What is the useState hook?

Handling Events

21. How do you handle events in React?

22. What is the difference between event handling in React and in HTML?

23. How can you prevent the default action of an event in React?

24. What are synthetic events in React?

25. How do you bind methods in React?

Conditional Rendering

26. What is conditional rendering in React?

27. How do you implement conditional rendering using if statements?

28. What is a ternary operator, and how is it used in React?

29. How can you use logical AND (&&) for conditional rendering?

30. What is the Fragment component, and how is it useful?

Lists and Keys

31. How do you render a list in React?

32. What are keys in React, and why are they important?

33. How do you generate unique keys for a list?

34. What happens if you don't provide keys to a list of elements?

35. How can you update a list of items in React?

Forms and Inputs

36. How do you create a form in React?

37. What are controlled components?

38. How do you manage form input values using state?

39. What is the difference between controlled and uncontrolled components?

40. How do you handle form submission in React?

Hooks

41. What are hooks in React?

42. What is the useEffect hook?

43. How do you manage side effects with useEffect?

44. What is the purpose of the useContext hook?

45. What is the useReducer hook used for?

Performance Optimization

46. What is React.memo?

47. How can you optimize a React application?

48. What is code splitting in React?

49. How can you avoid unnecessary re-renders in React?

50. What are lazy loading and suspense in React?

Routing

51. What is React Router?

52. How do you implement routing in a React application?

53. What is the purpose of BrowserRouter?

54. What are route parameters in React Router?

55. How do you create nested routes in React Router?

Context API

56. What is the Context API in React?

57. How do you create a context in React?

58. How can you consume a context in a component?

59. What are the advantages of using the Context API?

60. What is context provider and context consumer?

Error Handling

61. How do you handle errors in React?

62. What is the purpose of componentDidCatch?

63. How can you implement error boundaries in React?

64. What happens if an error occurs in a functional component?

65. How do you display fallback UI during an error?

Advanced Topics

66. What is prop drilling?

67. How can you avoid prop drilling?

68. What is the purpose of React.StrictMode?

69. How can you test React components?

70. What is the difference between shallow and deep rendering?

Accessibility

71. How do you ensure accessibility in a React application?

72. What is ARIA, and how is it used in React?

73. How do you manage focus in a React application?

74. What is the role of alt attributes in images?

75. How can you create accessible forms in React?

Miscellaneous

76. What is the difference between client-side and server-side rendering?

77. How do you use third-party libraries in React?

78. What is the purpose of keys in lists?

79. What is the purpose of the defaultProps property?

80. What are refs in React?

Best Practices

81. What are some best practices for structuring a React application?

82. How do you handle state management in a large application?

83. What is the significance of using functional components over class components?

84. How do you organize styles in a React project?

85. What tools do you use for debugging React applications?

Version Control and Deployment

86. How do you set up a React project using Create React App?

87. What is Webpack, and how is it used in React?

88. How do you deploy a React application?

89. What are the benefits of using TypeScript with React?

90. How can you set up a CI/CD pipeline for a React app?

Collaboration and Communication

91. How do you collaborate with designers in a React project?

92. What is the role of documentation in React projects?

93. How do you gather requirements for a React project?

94. What techniques do you use for effective team communication?

95. How do you handle code reviews in a React project?

Future Trends and Learning

96. What are the latest features in the most recent React version?

97. How do you stay updated with React developments?

98. What are some common mistakes to avoid when learning React?

99. What resources do you recommend for learning React?

100. What are your thoughts on the future of React?

General React Questions

1. What is React?

o React is an open-source JavaScript library used for building user interfaces, particularly single-page applications. It allows developers to create large web applications that can change data without reloading the page.

2. What are the main features of React?

o Key features include:

§ Component-Based Architecture: UI is divided into reusable components.

§ Virtual DOM: React maintains a virtual representation of the DOM, which improves performance by minimizing direct DOM manipulation.

§ One-way Data Binding: Data flows in a single direction, making it easier to understand the flow of data.

§ JSX: A syntax extension that allows writing HTML-like code within JavaScript.

3. What is the difference between React and other JavaScript frameworks?

o React focuses on building user interfaces, while frameworks like Angular and Vue provide more comprehensive solutions that include state management, routing, and other functionalities. React is more flexible and often requires integrating other libraries for these features.

4. What is a React component?

o A React component is a reusable piece of UI that encapsulates its structure, behavior, and styles. Components can be functional or class-based.

5. How do you create a functional component in React?

o A functional component is a JavaScript function that returns JSX. Example:

jsx

Copy code

function MyComponent() {

return <h1>Hello, World!</h1>;

}

6. What is a class component in React?

o A class component is defined using a JavaScript class and must extend React.Component. It can have lifecycle methods and state. Example:

class MyComponent extends React.Component {

render() {

return <h1>Hello, World!</h1>;

}

7. What is JSX?

o JSX (JavaScript XML) is a syntax extension that allows writing HTML-like code within JavaScript. It makes it easier to create React elements.

8. Why is JSX used in React?

o JSX makes it easier to visualize the UI structure while writing code. It also allows embedding JavaScript expressions within HTML-like syntax.

9. What are props in React?

o Props (short for properties) are a way of passing data from a parent component to a child component. They are read-only and allow for component customization.

10. How do you pass props to a component?

o Props are passed to components like attributes in HTML. Example:

Component Lifecycle

11. What is the component lifecycle in React?

o The component lifecycle refers to the series of methods that are called at different stages of a component's existence, including mounting, updating, and unmounting.

12. What are the different lifecycle methods in class components?

o Key lifecycle methods include:

§ componentDidMount

§ componentDidUpdate

§ componentWillUnmount

13. What is the purpose of componentDidMount?

o componentDidMount is called after a component is mounted. It is often used for making API calls or setting up subscriptions.

14. What is componentWillUnmount used for?

o componentWillUnmount is called just before a component is unmounted and can be used for cleanup tasks, such as cancelling network requests or removing event listeners.

15. What is the difference between componentDidUpdate and componentWillUpdate?

o componentDidUpdate is called after a component updates, while componentWillUpdate (deprecated) is called before an update occurs.

State Management

16. What is state in React?

o State is a built-in object that allows components to create and manage their own data. It can change over time, causing re-renders.

17. How do you set state in a class component?

o State is set using the setState method. Example:

this.setState({ count: this.state.count + 1 });

18. What is the setState method?

o setState is used to update the state of a component. It schedules an update and merges the new state with the current state.

19. How do you initialize state in a functional component?

o State is initialized using the useState hook. Example:

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

20. What is the useState hook?

o useState is a React hook that allows functional components to manage state. It returns an array with the current state and a function to update it.

Handling Events

21. How do you handle events in React?

o Events are handled by passing a function as a prop to an event handler. Example:

<button onClick={this.handleClick}>Click me</button>

22. What is the difference between event handling in React and in HTML?

o In React, event handlers are camelCase properties and are passed as functions, while in HTML, they are lowercase and use strings.

23. How can you prevent the default action of an event in React?

o Use the preventDefault method in the event handler. Example:

handleSubmit = (event) => {

event.preventDefault();

// Handle form submission

};

24. What are synthetic events in React?

o Synthetic events are cross-browser wrappers around native events that ensure consistent behavior across different browsers.

25. How do you bind methods in React?

o Methods can be bound in the constructor or using arrow functions. Example:

constructor() {

super();

this.handleClick = this.handleClick.bind(this);

}

Conditional Rendering

26. What is conditional rendering in React?

o Conditional rendering is the process of rendering different components or elements based on certain conditions.

27. How do you implement conditional rendering using if statements?

o You can use if statements within the render method to conditionally return elements. Example:

render() {

if (this.state.isLoggedIn) {

return <h1>Welcome!</h1>;

}

return <h1>Please log in.</h1>;

}

28. What is a ternary operator, and how is it used in React?

o The ternary operator is a shorthand for an if-else statement. It is often used for conditional rendering. Example:

{this.state.isLoggedIn ? <h1>Welcome!</h1> : <h1>Please log in.</h1>}

29. How can you use logical AND (&&) for conditional rendering?

o You can use the logical AND operator to render an element if a condition is true. Example:

{this.state.isLoggedIn && <h1>Welcome!</h1>}

30. What is the Fragment component, and how is it useful?

o Fragment is a component that allows grouping multiple elements without adding extra nodes to the DOM. Example:

<h1>Title</h1>

<p>Description</p>

</Fragment>

Lists and Keys

31. How do you render a list in React?

o You can render a list using the map method. Example:

const items = ['Apple', 'Banana', 'Cherry'];

const listItems = items.map((item) => <li key={item}>{item}</li>);

32. What are keys in React, and why are they important?

o Keys are unique identifiers for elements in a list. They help React identify which items have changed, are added, or are removed, enhancing performance.

33. How do you generate unique keys for a list?

o Unique keys can be generated using unique IDs from your data or by using the index of the item in the array (not recommended if the list can change).

34. What happens if you don't provide keys to a list of elements?

o If keys are not provided, React will use the index as a key, which can lead to performance issues and incorrect behavior when reordering or modifying the list.

35. How can you update a list of items in React?

o To update a list, you create a new array with the desired changes and update the state. Example:

const updatedItems = this.state.items.map(item => {

if (item.id === idToUpdate) {

return { ...item, name: newName };

}

return item;

});

this.setState({ items: updatedItems });

Forms and Inputs

36. How do you create a form in React?

o A form can be created using standard HTML form elements. Example:

<button type="submit">Submit</button>

</form>

37. What are controlled components?

o Controlled components are form elements whose value is controlled by React state. They use value and onChange props to manage user input.

38. What are uncontrolled components?

o Uncontrolled components manage their own state internally and are not controlled by React. You can access their values via refs.

39. How do you handle form submissions in React?

o Form submissions are handled by attaching an event handler to the form's onSubmit event. Example:

handleSubmit = (event) => {

event.preventDefault();

// Handle form submission

};

40. How do you implement controlled inputs?

o Controlled inputs are implemented by binding the input value to state and updating the state on input changes. Example:

<input type="text" value={this.state.inputValue} onChange={(e) => this.setState({ inputValue: e.target.value })} />

Hooks

41. What are hooks in React?

o Hooks are functions that allow functional components to use state and other React features. They enable state management and side effects in functional components.

42. What is the useEffect hook?

o useEffect is a hook that allows you to perform side effects in functional components, such as data fetching, subscriptions, or manually changing the DOM.

43. How do you use the useEffect hook?

o useEffect takes two arguments: a function for the side effect and an optional dependency array. Example:

useEffect(() => {

// Side effect code here

}, [dependencies]);

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

o The dependency array determines when the effect runs. If any value in the array changes, the effect runs again. If empty, it runs once on mount.

45. What is the useContext hook?

o useContext is a hook that allows you to access context values in functional components, making it easier to share data across the component tree.

Routing

46. What is React Router?

o React Router is a library for managing routing in React applications. It allows you to define routes and navigate between different components based on the URL.

47. How do you set up routing in a React application?

o You set up routing by installing react-router-dom and using BrowserRouter, Route, and Link components. Example:

import { BrowserRouter as Router, Route, Link } from 'react-router-dom';

</Router>

48. What is a route in React Router?

o A route defines a mapping between a URL path and a component. When the path matches the current URL, the specified component is rendered.

49. What is the Link component used for?

o The Link component is used for navigating between routes in a React application without refreshing the page.

50. What is a nested route in React Router?

o A nested route is a route defined inside another route. It allows rendering child components based on the parent route's path.

Performance Optimization

51. How can you optimize performance in a React application?

o Performance can be optimized by:

§ Using React.memo for functional components to prevent unnecessary re-renders.

§ Implementing lazy loading with React.lazy and Suspense.

§ Using useCallback and useMemo hooks to memoize functions and values.

52. What is React.memo?

o React.memo is a higher-order component that memoizes a functional component, preventing re-renders if the props haven't changed.

53. What is the React.lazy function?

o React.lazy allows you to dynamically import components, enabling code-splitting and reducing the initial bundle size.

54. What is Suspense in React?

o Suspense is a component that allows you to display a fallback UI (like a loading spinner) while a lazy-loaded component is being fetched.

55. How do you use useCallback and useMemo?

o useCallback returns a memoized callback function, while useMemo returns a memoized value. They help optimize performance by preventing unnecessary re-computation. Example:

const memoizedValue = useMemo(() => computeExpensiveValue(a, b), [a, b]);

const memoizedCallback = useCallback(() => { doSomething(a, b); }, [a, b]);

Error Handling

56. How can you handle errors in a React application?

o Errors can be handled using error boundaries. You can create a class component that implements componentDidCatch to catch errors in its child components.

57. What is an error boundary?

o An error boundary is a React component that catches JavaScript errors in its child components, logs them, and displays a fallback UI instead of crashing the entire component tree.

58. How do you create an error boundary?

o You create an error boundary by defining a class component with componentDidCatch and getDerivedStateFromError methods. Example:

class ErrorBoundary extends React.Component {

constructor(props) {

super(props);

this.state = { hasError: false };

}

static getDerivedStateFromError(error) {

return { hasError: true };

}

componentDidCatch(error, info) {

// Log error information

}

render() {

if (this.state.hasError) {

return <h1>Something went wrong.</h1>;

}

return this.props.children;

}

59. How do you log errors in React?

o You can log errors using console.error or by sending error information to an error logging service in the componentDidCatch method.

60. What is the purpose of the getDerivedStateFromError method?

o getDerivedStateFromError is used to update the state in response to an error, allowing the error boundary to render a fallback UI.

Testing

61. How do you test React components?

o React components can be tested using libraries like Jest and React Testing Library, which provide tools for rendering components, simulating events, and asserting behaviors.

62. What is Jest?

o Jest is a JavaScript testing framework used for unit testing React applications. It provides features like snapshot testing and mocking.

63. What is React Testing Library?

o React Testing Library is a library for testing React components by focusing on user interactions and behaviors, promoting best practices for testing.

64. How do you write a simple test for a React component?

o Example test using React Testing Library:

import { render, screen } from '@testing-library/react';

import MyComponent from './MyComponent';

test('renders hello message', () => {

render(<MyComponent />);

const linkElement = screen.getByText(/Hello, World!/i);

expect(linkElement).toBeInTheDocument();

});

65. What is snapshot testing?

o Snapshot testing is a method of testing the rendered output of a component by comparing it to a previously saved snapshot, helping to detect changes.

Context and State Management

66. What is the Context API in React?

o The Context API is a feature that allows you to share data between components without passing props through every level of the tree, making it easier to manage global state.

67. How do you create a context in React?

o You can create a context using React.createContext(). Example:

const MyContext = React.createContext();

68. How do you provide and consume context?

o You provide context using a Provider component and consume it using the useContext hook or Context.Consumer. Example:

<MyContext.Provider value={value}>

</MyContext.Provider>

69. What is Redux?

o Redux is a predictable state container for JavaScript applications, often used with React for managing global state. It uses actions, reducers, and a single store.

70. How does Redux differ from the Context API?

o Redux provides a more structured approach with middleware, DevTools, and a single source of truth, while the Context API is simpler and suitable for smaller applications.

Advanced Topics

71. What are higher-order components (HOCs)?

o HOCs are functions that take a component and return a new component with added functionality, promoting code reuse and separation of concerns.

72. What is a render prop?

o A render prop is a technique for sharing code between components by passing a function as a prop that returns a React element.

73. How do you implement code splitting in a React application?

o Code splitting can be implemented using React.lazy and Suspense, allowing you to load components dynamically based on routes or conditions.

74. What is server-side rendering (SSR)?

o SSR is the process of rendering a React application on the server and sending the fully rendered page to the client, improving SEO and performance.

75. What is static site generation (SSG)?

o SSG is a method of pre-rendering pages at build time, allowing for fast loading and improved performance, often used with frameworks like Next.js.

Conclusion

This list covers fundamental concepts and practical knowledge necessary for React development.

React.js interview questions along with their answers

React.js interview questions

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