React Js interview questions
By admin | 9 months ago
1. What is React and why is it used?
Answer: React is a declarative, efficient, and flexible JavaScript library for building user interfaces. It enables developers to create large web applications that can change data, without reloading the page. Its main goal is to be fast, scalable, and simple. It works only on user interfaces in the application. This corresponds to the view in the MVC template. It can be used with a combination of other JavaScript libraries or frameworks, such as Angular JS in MVC.
2. Explain JSX.
Answer: JSX stands for JavaScript XML. It is a syntax extension for JavaScript recommended by React for writing React components. It looks like HTML but it's actually JavaScript. JSX makes it easier to write and add HTML in React.
3. What are components in React?
Answer: Components are the building blocks of any React application, and a single app usually consists of multiple components. They are essentially JavaScript classes or functions that can accept inputs, called "props", and return React elements that describe how a section of the UI should appear.
4. What are props in React?
Answer: Props (short for properties) are a way of passing data from parent to child components. They are read-only and help to render dynamic data in components.
5. Explain the state in React.
Answer: The state is a built-in React object used to contain data or information about the component. A component’s state can change over time, and whenever it changes, the component re-renders.
6. What are lifecycle methods in React?
Answer: Lifecycle methods are special methods each component can have that allow us to hook into the views when specific conditions happen (e.g., when the component is created, added to the DOM, updated, or destroyed).
7. What is the significance of keys in React lists?
Answer: Keys help React identify which items have changed, are added, or are removed from the lists. It’s a way to give an identity to the elements in the lists created dynamically.
8. Explain the virtual DOM.
Answer: 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.
9. Describe the difference between a controlled component and an uncontrolled component.
Answer: A controlled component takes its current value through props and notifies changes through callbacks like onChange. An uncontrolled component keeps its own internal state and interacts with the DOM directly using refs.
10. What are higher-order components (HOC)?
Answer: Higher-order components are a pattern where a function takes a component and returns a new component, thereby adding additional functionalities to the component.
11. What is React Router?
Answer: React Router is a standard library system built on top of React that allows the implementation of dynamic routing in a web app.
12. Explain hooks in React.
Answer: Hooks are a new addition in React 16.8 that allow you to use state and other React features without writing a class. Examples include useState, useEffect, useContext, etc.
13. What is Redux?
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.
14. How do you handle events in React?
Answer: In React, events are handled using camelCase syntax. The event handler is a function that is passed to the component and is triggered by an event.
15. What is the use of the useEffect Hook?
Answer: The useEffect Hook lets you perform side effects in function components. It is a close replacement for componentDidMount, componentDidUpdate, and componentWillUnmount in class components.
16. Explain the concept of lifting state up in React.
Answer: Lifting state up is the process of moving the state up to the parent component to keep the child components in sync or to share state between sibling components.
17. What are pure components?
Answer: Pure components are components that do not re-render unless there is a change in the state or props. They can improve the performance of applications.
18. Describe React’s reconciliation algorithm.
Answer: Reconciliation is the process through which React updates the DOM. When a component’s state changes, React first changes the component’s state and then compares the newly returned element with the previous one. If there are differences, React updates the DOM.
19. What is context in React?
Answer: Context provides a
way to pass data through the component tree without having to pass props down manually at every level.
20. How do you create a React app?
Answer: You can create a React app by using create-react-app, which sets up a modern web development environment with no configuration.
21. What is the difference between React and Angular?
Answer: React is a library that is mainly focused on building UI components, while Angular is a full-fledged MVC framework. React uses virtual DOM and one-way data binding while Angular operates on real DOM and two-way data binding.
22. How can you improve the performance of a React application?
Answer: Performance can be improved by avoiding unnecessary re-renders, using shouldComponentUpdate or React.memo, code-splitting, lazy loading components, optimizing assets, and using the Production build.
23. What is memoization in React?
Answer: Memoization is an optimization technique that makes React apps more efficient by storing the results of expensive function calls and returning the cached result when the same inputs occur again.
24. Explain the differences between a class component and a functional component.
Answer: Class components allow you to use additional features such as state and lifecycle methods. Functional components are simpler and are used for components that only require a render method without state or lifecycle methods, though Hooks now allow functional components to use state and other React features.
25. What are fragments in React?
Answer: Fragments let you group a list of children without adding extra nodes to the DOM.
26. How does React handle forms?
Answer: React handles forms by keeping the form data in the component's state. Form elements become controlled components, with their state managed by React.
27. What is PropTypes in React?
Answer: PropTypes is a library that helps in validating the types of props passed to the components. It ensures that your components use the correct data type and passes the right data.
28. Explain the forwardRef API in React.
Answer: The forwardRef API in React allows you to pass a ref down to a child component. This is particularly useful in higher-order components or when the child component needs to be directly accessed in the parent component.
29. What are error boundaries in React?
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.
30. How can you integrate React with other frameworks, such as Angular or Vue?
Answer: React can be integrated with other frameworks by treating it as a microfrontend. This involves creating a React component and exporting it as a custom element or using specific libraries designed for integration like single-spa.
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