React.js MCQ Questions Answers

1. What pattern is being used in this code? const { tree, lake } = nature;

2. Why might you use a React.ref?

3. Which language can you not use with React?

4. When using the React Developer Tools Chrome extension, what does it mean if the React icon is red?

5. What might you use webpack for?

6. When does the componentDidMount function fire?

7. You want to disable a button so that it does not emit any events onClick. Which prop do you use to acomplish this?

8. What syntax do you use to create a component in React?

9. How might you check property types without using Flow or TypeScript?

10. What is the JavaScript syntax extension that is commonly used to create React elements?

11. Which tool is not part of Create React App?

12. What is the browser extension called that React developers use to debug applications?

13. What command can you use to generate a React project?

14. React is an open-source project but is maintained by which company?

15. What is the process of deciding whether an update is necessary?

16. How would you generate the boilerplate code for a new app that you are building to collect underpants?

17. You are rendering a list with React when this warning appears in the console: "Warning: Each child in a list should have a unique 'key' prop." How do you fix this issue?

18. What is the first file loaded by the browser in a basic React project?

19. What is this pattern called? const [count, setCount] = useState(0);

20. When using a portal, what is the second argument?ReactDOM.createPortal(x, y);

21. Describe what is happening in this code? const { name: firstName } = person;

22. What is setCount? const [count, setCount] = useState(0);

23. When using a portal, what is the first argument? ReactDOM.createPortal(x, y);

24. What is the first argument, x, that is sent to the createElement function?React.createElement(x, y, z);

25. Why should you use React Router's Link component instead of a basic tag in React?

26. Consider the following component. What is the default color for the star? const Star = ({ selected = false }) => ;

27. Which function is used to update state variables in a React class component?

28. Per the following code, when is the Hello component displayed? const greeting = isLoggedIn ? : null;

29. Which choice will not cause a React component to rerender?

30. Which library does the fetch() function come from?

31. Which answer best describes a function component?

32. Currently, handleClick is being called instead of passed as a reference. How do you fix this? Click this

33. What can you use to wrap Component imports in order to load them lazily?

34. Q42. Which Hook could be used to update the document's title?

35. How do you set a default value for an uncontrolled form field?

36. What package contains the render() function that renders a React element tree to the DOM?

37. Why is it a good idea to pass a function to setState instead of an object?

38. What is sent to an Array.map() function?

39. All React components must act like _ with respect to their props.

40. React components are composed to create a user interface. How are components composed?

41. In which lifecycle method do you make requests for data in a class component?

42. What do you call a React component that catches JavaScript errors anywhere in the child component tree?

43. To create a constant in JavaScript, which keyword do you use?

44. A representation of a user interface that is kept in memory and is synced with the "real" DOM is called what?

45. When using webpack, why would you need to use a loader?

46. Which of these terms commonly describe React applications?

47. Which attribute do you use to replace innerHTML in the browser DOM?

48. What is the children prop?

49. Why is it important to avoid copying the values of props into a component's state where possible?

50. When might you use React.PureComponent?

51. When do you use useLayoutEffect?

52. What can you use to handle code splitting?

53. Consider the following code from React Router. What do you call :id in the path prop?

54. Which props from the props object is available to the component with the following syntax?

55. Why might you use useReducer over useState in a React component?

56. What is the name of the tool used to take JSX and turn it into createElement calls?

57. How do you handle passing through the component tree without having to pass props down manually at every level?

58. What is the testing library most often associated with React?

59. If a function component should always render the same way given the same props, what is a simple performance optimization available for it?

60. If you want to import just the Component from the React library, what syntax do you use?

61. Can you render children components outside of a parent component?

62. How can you bind a function to a component instance? (Check all that apply.)

63. How you can provide an initial value for uncontrolled input?

64. _____ function is used for attaching event listeners on markup rendered by ReactDOMServer.

65. Can you access refs at a wrapped component in higher-order component technique?

66. Which of following is true about componentWillReceiveProps and getDerivedStateFromProps? (Check all that apply.)

67. What programming concept React use for DOM manipulation and updating?

68. Can you return multiple elements from a single component?

69. How can you stop event propagation in React? (Check all that apply.)

70. What happens when an error is not caught by error boundary?

71. What is the name of a pseudo-global variable that determines whether the environment is development or production?

72. What is a recommended way of sharing data between multiple components? (Check all that apply.)

73. What is a purpose of shallow rendering in React?

74. What is the difference between renderToString and renderToStaticMarkup ReactDOMServer's functions?

75. ____ is flux architecture component that receives actions and broadcast payloads to registered callbacks.

76. Which of the following are valid properties of the React Router's Route component? (Check all that apply.)

77. Jest and Enzyme are suggested libraries for what purpose?

78. How can type checking in React be done? (Check all that apply.)

79. When should you use React.PureComponent?

80. Which function is invoked just before render() during component update?

81. Which of the following is true about keys in React?

82. Static object allows the defining of static methods and also means that these methods do not have access to or ?

83. What is the difference between props and state?

84. Which of the following is true about React Library?Which of the following is true about React Library? (choose all that apply)

85. Can a single component render multiple nodes?

86. How can SyntheticEvent be stopped?

87. The defaultValue and defaultChecked props are only used during initial __?

88. What will be removed by JSX from the string inside node?

89. Which value will render nothing? (Check all that apply)

90. Is it possible to create ReactJS components without JSX?

91. Which of the following is a correct JSX code?

92. Which of the following are valid properties of the input node in ReactJS?

93. What is true about forceUpdate method in component?

94. What is true about context inside a component?

95. What is the correct value of the children proptype? propTypes: { children: React.PropTypes.string.isRequired }

96. Which of the following is the correct data flow in a flux application?

97. What can be stored in the Stores while building with flux architecture?

98. Which statement is valid for ReactDOM.findDOMNode() function?

99. What values can be passed as children of a ReactJs component?

100. Which function is invoked just before render() during initial rendering?