Top 50 ReactJS Interview Questions and Answers with Examples

9. How can you embed two or more components into one?

Components are the building blocks of a React application’s UI. They can be nested in each other. Components whose output is influenced by other components are referred to as composite components. Here’s an example:

function App() {

return ( <div> <Header /> <Main /> <Footer /> </div> );

}

10. What is the virtual DOM and how does it work?

The virtual DOM (VDOM) is a programming concept where an ideal, or “virtual”, representation of a UI is kept in memory and synced with the “real” DOM by a library such as ReactDOM. This process is called reconciliation. It provides a simple programming model, better performance, and allows React to coexist with other libraries that use the DOM.

11. What is the significance of keys in React?

Keys in React are unique identifiers that help React recognize which items have been changed, added, or removed. They’re used in cases where the item order can change, such as in dynamic lists, to improve performance.

12. How can you update the state of a component?

The state of a component in React can be updated using the setState() function. This function triggers UI updates and is typically used within event handlers.

13. What are synthetic events in React?

Synthetic events are wrappers around browser’s native events, having the same interface as native events. React uses synthetic events to ensure that the events have consistent properties across different browsers.

14. Explain the use of refs in React.

Refs in React provide a way to access DOM nodes or React elements created in the render method. They can be useful for a variety of tasks including managing focus, text selection, or media playback.

15. Can you explain the concept of controlled components?

In React, a controlled component is a component that renders form elements and controls them by keeping the form data in the component’s state. In a controlled component, the state is the single source of truth for the form data.

16. What is the Context API in React?

The Context API is a component structure provided by React that enables you to pass props directly from a parent component to a child component without having to pass them through other parent components. This is particularly beneficial for props that are needed by many components within an application.

17. How does conditional rendering work in React?

Conditional rendering in React works the same way conditions work in JavaScript. Different components can be rendered on the DOM depending on the state or props. This is done using JavaScript operators like if or the conditional (ternary) operator to create elements representing the current state.

18. What is React Router?

React Router is a powerful routing library built on top of React, which helps in adding new screens and flows to the application. This keeps the URL in sync with data that’s being displayed on the web page. It maintains the standard structure and behavior of the application and is used for developing single page web applications.

19. What is the significance of render props in React?

Render Props is a technique in React where a child component is given a function prop that returns a React element. This technique makes the component’s behavior more reusable.

20. Can you explain the concept of Higher Order Components (HOC)?

Higher Order Components (HOC) is an advanced way of reusing component logic. Simply, an HOC is a function that accepts a component and returns a new component. It’s a pattern that is derived from React’s compositional nature.

21. What is React Fragment and why do we use it?

React Fragments let you group a list of children without adding extra nodes to the DOM. It’s a common pattern in React where a component returns multiple elements. Fragments allow you to wrap a list of children without adding unnecessary nodes to the DOM.

22. What are the advantages of React?

React is popular due to its simplicity, performance, and scalability. It enables developers to create complex UIs from small and isolated pieces of code called “components”. The virtual DOM improves app performance and the support for server-side rendering is beneficial for SEO.

23. What are the limitations of React?

React is a library, not a full-fledged framework. Its library ecosystem is vast and can be complicated for beginners to understand. Also, the frequent updates to React may require developers to constantly adapt to changes.

24. How does event handling work in React?

Event handling in React elements is very similar to handling events on DOM elements. The only difference is React events are named using camelCase, rather than lowercase. With JSX you pass a function as the event handler, rather than a string.

25. How to apply inline styles in React?

Inline styles are not specified as a string. Instead they are specified with an object whose key is the camelCased version of the style name, and whose value is the style’s value, usually a string. Here is an example:

    function App() {
        const divStyle = {
            color: 'blue',
            backgroundImage: 'url(' + imgUrl + ')'
        };

        return

; }

26. What is prop drilling and how can you avoid it?

Prop drilling is the process of getting data to parts of the React Component tree. This involves passing props down from a parent component to a child component, and it can become troublesome for certain designs. Context API or Redux can be used to avoid prop drilling.

27. How do you create a ref in React?

Refs are created using `React.createRef()` method and attached to React elements via the `ref` attribute. Refs are commonly assigned to an instance property when a component is constructed so they can be referenced throughout the component.

28. How do you pass a parameter to an event handler or callback?

You can use an arrow function to wrap around an event handler and pass parameters, like this:

     this.handleClick(id, e)}>Click me
    

29. Explain the lifecycle methods of React components in detail.

React component lifecycle can be divided into three main phases: Mounting, Updating, and Unmounting.

Mounting: Methods called in this phase are: constructor(), static getDerivedStateFromProps(), render(), and componentDidMount().

Updating: If a component is updated, the methods called in this order: static getDerivedStateFromProps(), shouldComponentUpdate(), render(), getSnapshotBeforeUpdate(), and componentDidUpdate().

Unmounting: The method called in this phase is componentWillUnmount().

30. How does React handle forms?

Forms in React are similar to HTML forms, with a bit of a difference. The React form’s state is typically managed within the React component using it. Whenever the state changes, the component re-renders, providing the updated state to the rendered form.

31. What are error boundaries in React?

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.

32. What is the use of super(props) in a React component?

super(props) is used in the constructor of a React component. It makes the `this.props` reference available in the constructor and is necessary to access the parent’s properties.

33. Explain how lists work in React.

Lists in React are simply arrays of elements. We can render lists in React using the JavaScript function map(). Each item in a list should have a unique ‘key’ prop. The ‘key’ helps React identify which items have changed, are added, or are removed, enhancing performance.

34. What is a pure component in React?

Pure components in React are a simpler way to write functional components when they can be purely defined by props. A pure component ignores re-renders unless its own props have changed, as determined by a shallow comparison.

35. Explain the concept of lifting state up in React.

Lifting state up in React involves moving the state from child components to a common ancestor component. This enables synchronization and communication between child components through the common ancestor, helping avoid inconsistency between related components.

36. Explain the use of arrow functions in React.

Arrow functions are often used in React for their terse syntax and ‘this’ binding. They can make your React code cleaner and simpler. The ‘this’ inside arrow functions is lexically bound and allows the functions to access ‘this’ from the context they were created in.

37. How does React Router work?

React Router maintains a dynamic and responsive user interface synchronized with the URL. It uses the concept of declarative programming for routing which makes the code more readable and easier to maintain.

38. How to use innerHTML in React?

The property dangerouslySetInnerHTML is React’s replacement for using innerHTML in the browser DOM. In general, setting HTML from code is risky because it’s easy to inadvertently expose your users to a cross-site scripting (XSS) attack. So, only use dangerouslySetInnerHTML when necessary.

39. What are hooks in React?

React Hooks are functions that let you ‘hook into’ React state and lifecycle features from function components. Hooks don’t work inside classes — they let you use React without classes. They were introduced in React 16.8.

40. What is useState() in React?

The useState() is a Hook that allows you to add React state to function components. It accepts a single argument which is the initial state, and it returns an array containing the current state and a function to update it.

41. Explain the useEffect() hook in React.

useEffect() hook in React is used to perform side effects in function components. Side effects could be anything like data fetching, subscriptions, or manually changing the DOM. It serves the same purpose as componentDidMount(), componentDidUpdate(), and componentWillUnmount() in React classes.

42. How do you handle form submission in React?

In React, form submission can be handled by listening for the onSubmit event and getting form data from the state. The form fields update the state, and when the form is submitted, the data is sent to the server or processed in some way.

43. Explain prop types in React.

PropTypes in React allows you to specify the type and structure of the data your component needs to function properly. If an incoming prop doesn’t match what is expected, React will throw a warning in the JavaScript console during development.

44. What is the role of Redux Thunk in Redux?

Redux Thunk is a middleware that allows you to write action creators that return a function instead of an action. The function can be used to delay the dispatch of an action or to dispatch only if a certain condition is met.

45. What is a stateless component in React?

A stateless component is also known as a functional component. It is a component that simply receives props and renders them to the page. It doesn’t manipulate data, have internal state, or have a lifecycle. It’s essentially a JavaScript function that returns JSX.

46. What is the virtual DOM in React?

The virtual DOM (VDOM) is a programming concept where an ideal, or “virtual”, representation of a UI is kept in memory and synced with the “real” DOM. It’s a step that happens between the render function being called and the displaying of elements on the screen.

47. What is Redux in React?

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. In conjunction with React, it facilitates the transmission of data through components.

48. What is the role of React PropTypes?

PropTypes are used in React to specify the type of data a component should receive as props. They document what kind of properties a component accepts, which can be particularly helpful when the components get larger and more complex.

49. What is the difference between a class component and a functional component in React?

The main difference between a class component and a functional component is that the class component includes lifecycle methods and local state, while a functional component is simply a JavaScript function that returns JSX. However, with the introduction of React Hooks, functional components can now manage local state and side effects too.

50. What are React Portals?

React Portals provide a first-class way to render children into a DOM node that exists outside the DOM hierarchy of the parent component. This is used for modals, pop-ups, and tooltip-like widgets where you need to see the component over all the content and not be bound by the parent container’s styling.

In conclusion, ReactJS is a popular and potent library for building user interfaces, particularly single-page applications where you need a fast response time. These interview questions cover various aspects of React and provide a solid foundation for any ReactJS related interview. Remember, understanding the concepts behind these questions is crucial. Good luck with your interview preparation!