React js interview questions

What is React.js?

React.js is a popular JavaScript library used to create user interfaces (UI). It was developed by Facebook and now it is open-source. The main feature of React is “component-based architecture”, in which you can break your UI into small reusable components.

It uses the concept of “Virtual DOM”, which is faster in comparison to the actual DOM and updates the UI efficiently. React has “one-way data flow” which helps in managing data.

Some important features of React.js:

Component-Based Architecture – Makes development easier by breaking down UI into smaller reusable components.
Virtual DOM – Provides an optimized updating system for fast rendering performance.
JSX (JavaScript XML) – allows using HTML-like syntax that is written in JavaScript.
One-Way Data Binding – Data flow is unidirectional which makes the application predictable.
Hooks – Functional components allow the use of state and lifecycle methods.

In this article we will cover all most important React js interview questions and answers.

Basic React js interview questions and answers

1. What is React.js?
   React.js is a JavaScript library for building user interfaces, primarily used to create single-page applications (SPAs). It allows developers to build reusable UI components.
2. What are the key features of React?
   • Virtual DOM
   • JSX (JavaScript XML)
   • Component-based architecture
   • One-way data binding
   • Unidirectional data flow
   • State management
3. What is JSX?
   JSX is a syntax extension of JavaScript that allows HTML to be written inside JavaScript code. It makes the code more readable and expressive.
4. What is the difference between functional and class components?
   • Functional components are stateless and written as JavaScript functions.
   • Class components are stateful and written as ES6 classes.
5. What is the virtual DOM?
   The virtual DOM is a lightweight copy of the real DOM that React uses to update only the changed parts efficiently instead of re-rendering the entire DOM.
6. What are props in React?
   Props (short for properties) are read-only data passed from a parent component to a child component to configure and customize its behavior.
7. What is state in React?
   State is a built-in object used to manage component-specific data that can change over time and cause the component to re-render.
8. What are React hooks?
   Hooks are functions that allow functional components to use state and lifecycle methods. Examples include useState, useEffect, and useContext.
9. What is the difference between state and props?
   • State: Managed within the component, mutable.
   • Props: Passed from parent to child, immutable.
10. What is the purpose of useState hook?
    useState is used to add state management to functional components. It returns a state variable and a function to update it.

Intermediate React.js Interview Questions

What is the significance of keys in React lists?
Keys help React identify which items have changed, are added, or removed in a list, improving performance and avoiding unnecessary re-renders.

What is the role of the useEffect hook?
useEffect is used to perform side effects in functional components, such as data fetching, subscriptions, or manually changing the DOM.

What is lifting state up in React?
Lifting state up refers to moving the state to a common ancestor component so that multiple child components can access and update it.

How does React handle forms?
React handles forms using controlled or uncontrolled components. Controlled components keep the form data in React state, while uncontrolled components use the DOM.

What is React context?
Context provides a way to pass data through the component tree without prop drilling.

What is prop drilling and how do you avoid it?
Prop drilling is passing data through multiple nested components unnecessarily. It can be avoided using context API or state management libraries like Redux.

What is the difference between useEffect and componentDidMount?
componentDidMount is used in class components, while useEffect with an empty dependency array [] achieves the same effect in functional components.

What are higher-order components (HOCs)?
HOCs are functions that take a component as an argument and return a new component with additional functionalities.

How does conditional rendering work in React?
Conditional rendering can be achieved using ternary operators, && operators, or if statements inside JSX.

What is the importance of defaultProps?
defaultProps provides default values for props in case they are not provided by the parent component.

Advanced React.js Interview Questions

What is React Router?
React Router is a library used for handling navigation and routing in React applications.

How do you optimize performance in React applications?
Techniques include:

• Using React.memo for memoization
• Avoiding unnecessary re-renders
• Lazy loading components
• Using React’s PureComponent

What is server-side rendering (SSR)?
SSR renders React components on the server and sends HTML to the client, improving performance and SEO.

What is the difference between SSR and CSR?

• SSR: Initial HTML is generated on the server.
• CSR: Entire content is rendered in the browser using JavaScript.

What is React Fiber?
React Fiber is the reconciliation engine in React that optimizes rendering by breaking tasks into units of work.

What are controlled and uncontrolled components?

• Controlled: Managed by React state.
• Uncontrolled: Managed by the DOM.

How does React handle state management?
State can be managed using React’s built-in useState, useReducer, Context API, or external libraries like Redux and Zustand.

What is the purpose of the useReducer hook?
useReducer is an alternative to useState for handling more complex state logic.

What are React Portals?
Portals allow rendering children components outside their parent DOM hierarchy.

What are React Fragments?
Fragments allow grouping multiple elements without adding extra DOM nodes.

React.js with Redux Questions

What is Redux?
Redux is a predictable state container for JavaScript applications that helps manage application state centrally.

What are the core principles of Redux?

• Single source of truth
• State is read-only
• Changes are made using pure functions (reducers)

What are actions in Redux?
Actions are plain JavaScript objects that describe the intention to change the state.

What is a reducer in Redux?
A reducer is a pure function that determines how state changes in response to actions.

What are middleware in Redux?
Middleware allows side effects like asynchronous operations in Redux, such as redux-thunk or redux-saga.

React Performance Optimization Questions

What is memoization in React?
Memoization optimizes rendering by caching component results and reusing them when inputs are unchanged.

How does React.memo work?
React.memo is a higher-order component that prevents re-rendering of a component if its props have not changed.

What is the purpose of the useCallback hook?
useCallback memoizes callback functions to prevent unnecessary re-renders.

What is the purpose of the useMemo hook?
useMemo memoizes computation-heavy values and returns cached results.

Miscellaneous Questions

How do you handle authentication in React?
Authentication can be handled using tokens (JWT), session storage, or third-party services like Firebase.

How do you test React components?
React components can be tested using testing libraries such as Jest, React Testing Library, and Enzyme.

What is lazy loading in React?
Lazy loading is the technique of loading components only when needed using React.lazy() and Suspense.

What is a React custom hook?
A custom hook is a function that encapsulates logic and can be reused across components.

How does React handle error boundaries?
Error boundaries are special React components that catch JavaScript errors anywhere in the component tree.

Here we ready 100 Imp questions for you.

Basic React.js Interview Questions

1. What is React?
2. What are the advantages of using React?
3. What is the difference between React and Angular?
4. What is the significance of JSX in React?
5. Can browsers read JSX directly?
6. What is the difference between elements and components in React?
7. How do you create a component in React?
8. Explain the concept of React’s virtual DOM.
9. What is the significance of the key prop in React?
10. How do you create a functional component in React?
11. How do you create a class component in React?
12. What are React events, and how do they differ from regular HTML events?
13. What is the difference between onClick in HTML and React?
14. What is the difference between a controlled and an uncontrolled component?
15. What are refs in React, and how are they used?
16. What is the purpose of default props in React?
17. What is prop drilling in React, and how can it be avoided?
18. What is the difference between state and props?
19. Can you pass a function as props in React?
20. What are React fragments, and why are they used?

Intermediate React.js Interview Questions

21. What are React hooks?
22. Explain the use of the useState hook.
23. Explain the use of the useEffect hook.
24. What are the dependencies in the useEffect hook?
25. How do you fetch data in React using useEffect?
26. What are higher-order components (HOCs)?
27. What is the Context API in React?
28. How does React handle one-way data binding?
29. What is reconciliation in React?
30. What is the difference between local state and global state in React?
31. What is lazy loading in React?
32. How can React components be optimized?
33. What is React.memo, and when should you use it?
34. What is the significance of the useMemo hook?
35. Explain the purpose of the useCallback hook.
36. What are synthetic events in React?
37. What is the role of the children prop in React components?
38. What is component composition in React?
39. What is the difference between controlled and uncontrolled inputs?
40. How does error handling work in React?

Advanced React.js Interview Questions

41. What is the difference between Redux and Context API?
42. What are the main principles of Redux?
43. What is an action in Redux?
44. What is a reducer in Redux?
45. What is the Redux store, and how does it work?
46. What is the difference between Redux Thunk and Redux Saga?
47. How do you handle asynchronous operations in React?
48. What is server-side rendering (SSR) in React?
49. What are portals in React, and when should they be used?
50. What are hooks rules in React?
51. How does React handle reconciliation?
52. What is a PureComponent in React?
53. What is the difference between shouldComponentUpdate and React.memo?
54. What are error boundaries in React?
55. What are React render props?
56. How does component lifecycle work in React?
57. What are synthetic events in React?
58. How does React handle memory leaks in components?
59. How can you improve performance in large React applications?
60. How does concurrent rendering work in React?

React.js Performance Optimization Questions

61. What is code splitting in React?
62. How does React’s reconciliation algorithm improve performance?
63. What is the significance of using React.lazy() and Suspense?
64. How does the React Profiler work?
65. What are the common ways to avoid unnecessary re-renders in React?
66. What is debounce and throttle, and how can they be used in React?
67. What is tree-shaking in React applications?
68. What is the significance of shouldComponentUpdate in React?
69. How does React batch state updates?
70. What are micro-frontends in React applications?

React with TypeScript Questions

71. How do you define types for props in a React component using TypeScript?
72. What are interfaces and types in TypeScript React?
73. What is the difference between interface and type alias in TypeScript?
74. How do you type React functional components with TypeScript?
75. How do you type the state and props of a class component in TypeScript?
76. What is the use of useRef in TypeScript?
77. How can TypeScript improve the maintainability of React applications?
78. How do you define event types in React TypeScript?
79. How do you define default props in a TypeScript React component?
80. How do you use generics in React with TypeScript?

React Testing Questions

81. What are the different testing frameworks used with React?
82. What is React Testing Library?
83. How do you test React functional components?
84. What is snapshot testing in React?
85. What is Jest, and how is it used for React testing?
86. How do you mock API calls in React tests?
87. What is the purpose of fireEvent in React Testing Library?
88. How do you test form inputs in React?
89. What is the significance of act() in React tests?
90. What is the difference between shallow rendering and full rendering in React testing?

Miscellaneous React D

91. How do you handle authentication in React applications?
92. What is the difference between React and React Native?
93. How does React handle accessibility (a11y)?
94. What are React design patterns commonly used in production applications?
95. How do you manage multiple themes in a React application?
96. What are microservices, and how do they relate to frontend development in React?
97. What are progressive web applications (PWAs) in React?
98. What is the difference between React Router DOM and traditional routing?
99. How does React compare to frameworks like Vue.js and Svelte?
100. How does React handle real-time data updates using WebSockets?
     
     

So in this article we cover most of all important react.js questions

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