ReactJs Questions and Answers
1. What is ReactJS?
ReactJS is a JavaScript library used for building user interfaces. It allows developers to create reusable UI components and efficiently update them when the underlying data changes.
2. What is JSX?
JSX is a syntax extension for JavaScript used in React. It allows developers to write HTML-like code within JavaScript, making it easier to describe the structure and appearance of UI components.
3. What are the key features of ReactJS?
Some key features of ReactJS include virtual DOM, component-based architecture, one-way data flow, and the ability to create reusable UI components.
4. What is the virtual DOM in ReactJS?
The virtual DOM is a lightweight copy of the real DOM in memory. React uses it to efficiently update the user interface by comparing the virtual DOM with the actual DOM and applying only the necessary changes.
5. Explain the concept of components in ReactJS.
Components are the building blocks of React applications. They are reusable and independent units of UI logic that can be composed together to create complex user interfaces.
6. What is the difference between functional components and class components in ReactJS?
Functional components are stateless and written as regular JavaScript functions, while class components are stateful and written as ES6 classes. Functional components have a simpler syntax and are recommended for most use cases.
7. What is the purpose of state in ReactJS?
State is an object that represents the internal data of a component. It allows components to manage and update their own data, and when the state changes, React automatically re-renders the component.
8. How do you handle events in ReactJS?
In React, you can handle events by defining event handlers as methods within a component. These event handlers are triggered by user actions, such as button clicks or form submissions.
9. What is the significance of keys in ReactJS lists?
Keys are used in React to give each element in an array a stable identity. They help React identify which items have changed, been added, or been removed in a list, improving performance and reducing unnecessary re-rendering.
10. How do you pass data from a parent component to a child component in ReactJS?
Data can be passed from a parent component to a child component in React by using props. Props are passed as attributes to the child component, allowing it to access and use the data provided by the parent.
11. What is the role of the virtual DOM in React?
The virtual DOM is a lightweight copy of the actual DOM maintained by React. It allows React to efficiently update only the necessary parts of the UI by comparing the current and previous states of the virtual DOM.
12. What is the significance of the render() method in React components?
The render() method is responsible for rendering the component’s JSX markup. It gets called whenever the component needs to be updated or re-rendered.
13. What are React hooks?
React hooks are functions that allow you to use state and other React features in functional components. They provide a way to use stateful logic without writing a class.
14. Explain the useState hook in React.
The useState hook is used to add state to functional components. It returns an array with two elements: the current state value and a function to update that state.
15. What is the purpose of the useEffect hook?
The useEffect hook is used to perform side effects in functional components. It allows you to run code in response to component mounting, updating, or unmounting.
16. What is React Router?
React Router is a popular routing library for React applications. It enables navigation and URL handling within a single-page application.
17. How do you handle forms in React?
In React, form handling involves capturing form input values, managing their state, and handling form submission. This can be done using controlled components, where form inputs are tied to component state.
18. What is the purpose of the componentDidMount lifecycle method?
The componentDidMount method is a lifecycle method that gets called after a component has been rendered for the first time. It is commonly used to fetch data from an API or perform other initialization tasks.
19. How can you optimize performance in React?
Performance in React can be optimized by using shouldComponentUpdate or PureComponent to prevent unnecessary re-renders, implementing code splitting and lazy loading, and optimizing heavy operations.
20. What is Redux?
Redux is a state management library for JavaScript applications, often used with React. It provides a predictable and centralized way to manage application state.
21. Explain the concept of “unidirectional data flow” in React.
Unidirectional data flow is a design pattern in React where data flows in a single direction, from parent components to child components. This ensures that the application state remains predictable and easier to debug.
22. What is the significance of the key prop in React components?
The key prop is used to provide a unique identifier for elements in an array or list of components. It helps React efficiently update and re-render components when their order or structure changes.
23. What is the purpose of the shouldComponentUpdate method?
The shouldComponentUpdate method is a lifecycle method that allows you to control whether a component should re-render or not. By implementing this method, you can optimize performance by preventing unnecessary re-renders.
24. How can you handle events in React?
In React, event handling is done by providing event callbacks to JSX elements. These callbacks are functions that get executed when a specific event, such as a button click, occurs.
25. What is the significance of the setState method in React?
The setState method is used to update the state of a component. It takes an object or a callback function as an argument and triggers a re-render of the component with the updated state.
26. What are React refs?
React refs provide a way to access and interact with DOM elements or React components directly. They are commonly used to focus elements, trigger animations, or integrate with third-party libraries.
27. What is the purpose of the getDerivedStateFromProps method?
The getDerivedStateFromProps method is a static lifecycle method that allows a component to update its state based on changes in props. It is rarely used and often replaced by other methods.
28. What are React portals?
React portals provide a way to render a component’s children in a different part of the DOM hierarchy. They are useful for scenarios like modals or overlays, where the component needs to render outside its parent’s DOM tree.
29. What is the role of the componentWillUnmount lifecycle method?
The componentWillUnmount method is a lifecycle method that gets called right before a component is unmounted and removed from the DOM. It is used to clean up resources, event listeners, or subscriptions.
30. How do you perform server-side rendering with React?
Server-side rendering (SSR) with React involves rendering React components on the server and sending the pre-rendered HTML to the client. Libraries like Next.js provide built-in support for SSR.
31. What are React hooks?
React hooks are functions that allow you to use state and other React features in functional components. They provide a way to use stateful logic without writing a class.
32. Explain the useState hook in React.
The useState hook is used to add state to functional components. It returns an array with two elements: the current state value and a function to update that state.
33. What is the
purpose of the useEffect hook?
The useEffect hook is used to perform side effects in functional components. It allows you to run code in response to component mounting, updating, or unmounting.
34. What is the useContext hook used for?
The useContext hook is used to access the value of a context in a functional component. It allows you to consume context data without wrapping the component in a context consumer.
35. How can you optimize performance in React?
Performance in React can be optimized by using shouldComponentUpdate or PureComponent to prevent unnecessary re-renders, implementing code splitting and lazy loading, and optimizing heavy operations.
36. What is the purpose of the memo function in React?
The memo function is a higher-order component (HOC) that memoizes the result of a component rendering. It can be used to optimize functional components by preventing unnecessary re-renders.
37. What are React fragments?
React 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’s render method without using a container element.
38. What is the purpose of the useReducer hook?
The useReducer hook is used to manage state in functional components using a reducer function. It is an alternative to useState and is useful for managing complex state transitions.
39. How do you handle forms in React?
In React, form handling involves capturing form input values, managing their state, and handling form submission. This can be done using controlled components, where form inputs are tied to component state.
40. What are React hooks?
React hooks are functions that allow you to use state and other React features in functional components. They provide a way to use stateful logic without writing a class.
41. Explain the useState hook in React.
The useState hook is used to add state to functional components. It returns an array with two elements: the current state value and a function to update that state.
42. What is the purpose of the useEffect hook?
The useEffect hook is used to perform side effects in functional components. It allows you to run code in response to component mounting, updating, or unmounting.
43. What is the useContext hook used for?
The useContext hook is used to access the value of a context in a functional component. It allows you to consume context data without wrapping the component in a context consumer.
44. What is the purpose of the useMemo hook?
The useMemo hook is used to memoize the result of a function call, preventing unnecessary recalculations. It is useful for optimizing expensive computations within a functional component.
45. What are React fragments?
React 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’s render method without using a container element.
46. What is the purpose of the useCallback hook?
The useCallback hook is used to memoize a callback function, preventing unnecessary re-creations of the function. It is useful when passing callbacks to child components to avoid unnecessary re-renders.
47. What is the significance of the useRef hook?
The useRef hook is used to create a mutable reference that persists across component renders. It can be used to store a reference to a DOM element or a value that needs to persist between renders.
48. What are React portals?
React portals provide a way to render a component’s children in a different part of the DOM hierarchy. They are useful for scenarios like modals or overlays, where the component needs to render outside its parent’s DOM tree.
49. What is the purpose of the useLayoutEffect hook?
The useLayoutEffect hook is similar to useEffect, but it fires synchronously after all DOM mutations. It can be used when you need to perform measurements or access layout information immediately after a component has rendered.
50. How do you perform server-side rendering with React?
Server-side rendering (SSR) with React involves rendering React components on the server and sending the pre-rendered HTML to the client. Libraries like Next.js provide built-in support for SSR.
51. What are React hooks?
React hooks are functions that allow you to use state and other React features in functional components. They provide a way to use stateful logic without writing a class.
52. Explain the useState hook in React.
The useState hook is used to add state to functional components. It returns an array with two elements: the current state value and a function to update that state.
53. What is the purpose of the useEffect hook?
The useEffect hook is used to perform side effects in functional components. It allows you to run code in response to component mounting, updating, or unmounting.
54. What is the useContext hook used for?
The useContext hook is used to access the value of a context in a functional component. It allows you to consume context data without wrapping the component in a context consumer.
55. What is the purpose of the useMemo hook?
The useMemo hook is used to memoize the result of a function call, preventing unnecessary recalculations. It is useful for optimizing expensive computations within a functional component.
56. What are React fragments?
React 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’s render method without using a container element.
57. What is the purpose of the useCallback hook?
The useCallback hook is used to memoize a callback function, preventing unnecessary re-creations of the function. It is useful when passing callbacks to child components to avoid unnecessary re-renders.
58. What is the significance of the useRef hook?
The useRef hook is used to create a mutable reference that persists across component renders. It can be used to store a reference to a DOM element or a value that needs to persist between renders.
59. What are React portals?
React portals provide a way to render a component’s children in a different part of the DOM hierarchy. They are useful for scenarios like modals or overlays, where the component needs to render outside its parent’s DOM tree.
60. What is the purpose of the useLayoutEffect hook?
The useLayoutEffect hook is similar to useEffect, but it fires synchronously after all DOM mutations. It can be used when you need to perform measurements or access layout information immediately after a component has rendered.
61. What is the concept of code splitting in React?
Code splitting is a technique used to split your code into smaller chunks and load them on demand. It helps improve performance by reducing the initial load time of the application.
62. What is a higher-order component (HOC) in React?
A higher-order component is a function that takes a component as input and returns a new component with additional functionality. HOCs are used to share common functionality or to modify the behavior of components.
63. What is the purpose of the withRouter HOC in React Router?
The withRouter higher-order component is used in React Router to pass the router-related props (match, location, history) to a wrapped component. It allows access to routing information in components that are not directly rendered by a Route component.
64. What is the difference between shallow rendering and full rendering in React testing?
Shallow rendering in React testing allows you to render a component one level deep and assert on its output. Full rendering, on the other hand, renders the component and its child components recursively, allowing you to test the full component tree.
65. What is Redux?
Redux is a state management library for JavaScript applications, often used with React. It provides a predictable and centralized way to manage application state.
66. What are the core principles of Redux?
The core principles of Redux include a single source of truth (the state), state is read-only (actions are dispatched to modify the state), changes are made with pure functions (reducers), and changes are made using a predictable data flow.
67. What is an action in Redux?
An action in Redux is a plain JavaScript object that describes an intention to change the state. It must have a type property to indicate the type of action being performed.
68. What is a reducer in Redux?
A reducer in Redux is a pure function that takes the current state and an action as input, and returns a new state. It determines how the state should change in response to different actions.
69. What is a store in Redux?
A store in Redux is an object that holds the application state. It is responsible for dispatching actions, applying reducers, and notifying subscribers of state changes.
70. What is mapStateToProps in Redux?
mapStateToProps is a function used in React Redux to specify which part of the Redux state should be mapped to the props of a connected component. It allows the component to access the necessary state data.
71. What is mapDispatchToProps in Redux?
mapDispatchToProps is a function used in React Redux to define the actions that will be available as props in a connected component. It allows the component to dispatch actions to modify the state.
72. What is a middleware in Redux?
A middleware in Redux is a function that sits between the dispatching of an action and the moment it reaches the reducer. It allows you to intercept and modify actions or perform asynchronous operations.
73. What is the purpose of Redux Thunk middleware?
Redux Thunk is a middleware that allows you to write action creators that return functions instead of plain objects. This enables handling of asynchronous operations and delaying the dispatch of actions.
74. What is the purpose of Redux DevTools?
Redux DevTools is a browser extension that provides a development tool for inspecting and debugging Redux state changes. It allows you to track the dispatched actions, monitor the state, and time-travel through the application’s state history.
75. What is React Native?
React Native is a framework for building mobile applications using JavaScript and React. It allows you to write reusable UI components using React and target multiple platforms, including iOS and Android.
76. What are the advantages of using React Native?
Some advantages of using React Native include code reusability across platforms, a faster development cycle, a large ecosystem of community-driven libraries, and the ability to leverage existing web development skills.
77. What are the differences between React and React Native?
React is a JavaScript library for building user interfaces in web applications, while React Native is a framework for building mobile applications. React Native uses native components and APIs to render UI on mobile devices, while React renders components to the browser’s DOM.
78. What are the core components in React Native?
Core components in React Native are pre-built UI components provided by the framework, such as <View>, <Text>, <Image>, and <ScrollView>. They map to their respective native counterparts on iOS and Android.
79. What is Expo in React Native?
Expo is a set of tools and services for building, deploying, and managing React Native applications. It provides a simplified development workflow and access to various APIs and libraries without the need for native development tools.
80. What is the purpose of React Navigation in React Native?
React Navigation is a library for handling navigation in React Native applications. It provides a way to navigate between different screens, handle deep linking, and manage navigation state.
81. What is JSX in React?
JSX is a syntax extension for JavaScript that allows you to write HTML-like code within JavaScript. It is used in React to describe the structure and appearance of UI components.
82. What is the role of Babel in React development?
Babel is a JavaScript compiler that allows you to use modern JavaScript features, including JSX, in environments that do not support them natively. It is commonly used in React development to transform JSX and ES6+ code into browser-compatible JavaScript.
83. What is the virtual DOM in React?
The virtual DOM is a lightweight copy of the actual DOM in memory. React uses the virtual DOM to perform efficient updates and reconciliation, reducing the number of direct manipulations to the real DOM.
84. What is the reconciliation process in React?
The reconciliation process in React is the algorithm used to update the virtual DOM and determine the minimal set of changes needed to synchronize it with the actual DOM. It improves performance by minimizing DOM manipulations.
85. What is the purpose of React DevTools?
React DevTools is a browser extension or a standalone application that provides a development tool for inspecting and debugging React components. It allows you to view component hierarchies, inspect props and state, and track component re-renders.
86. What are React PropTypes?
React PropTypes are a way to specify the expected types and structure of props passed to a React component. They help catch potential bugs and provide documentation for component usage.
87. What is the purpose of React Fragments?
React 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’s render method without using a container element.
88. What are React keys and why are they important?
React keys are special attributes used to identify and differentiate sibling elements in a collection. They help React efficiently update and re-render components when their order or structure changes.
89. What is the role of the constructor in a React component?
The constructor is a lifecycle method in React components that is called when a component is being initialized and constructed. It is primarily used for initializing state and binding event handlers.
90. What is the significance of the render method in React components?
The render method is a required method in React components that returns the JSX or elements to be rendered to the DOM. It describes the structure and appearance of the component.
91. What is the purpose of the setState method in React?
The setState method is used to update the state of a component. It takes an object or a callback function as an argument and triggers a re-render of the component with the updated state.
92. What is the difference between controlled components and uncontrolled components in React?
Controlled components in React are components that manage their state through props and notify parent components of changes via callbacks. Uncontrolled components, on the other hand, manage their own state internally using references to DOM elements.
93. What is the purpose of the key prop in React lists?
The key prop is used in React lists to give each item a stable identity. It helps React efficiently update and re-render lists by providing a way to uniquely identify and track individual list items.
94. What are React hooks and why are they used?
React hooks are functions that allow you to use state and other React features in functional components. They provide a way to use stateful logic without writing a class. Hooks simplify code reuse and promote cleaner and more modular components.
95. What is the useEffect hook used for in React?
The useEffect hook is used to perform side effects in functional components. It allows you to run code in response to component mounting, updating, or unmounting.
96. What is the purpose of the useContext hook in React?
The useContext hook is used to access the value of a context in a functional component. It allows you to consume context data without wrapping the component in a context consumer.
97. What is the useReducer hook used for in React?
The useReducer hook is used to manage state in functional components using a reducer function. It is an alternative to useState and is useful for managing complex state transitions.
98. What is the useCallback hook used for in React?
The useCallback hook is used to memoize a callback function, preventing unnecessary re-creations of the function. It is useful when passing callbacks to child components to avoid unnecessary re-renders.
99. What is the purpose of the useMemo hook in React?
The useMemo hook is used to memoize the result of a function call, preventing unnecessary recalculations. It is useful for optimizing expensive computations within a functional component.
100. What is the useRef hook used for in React?
The useRef hook is used to create a mutable reference that persists across component renders. It can be used to store a reference to a DOM element or a value that needs to persist between renders.