React.js

Q1. If you want to import just the Component from the React library, what syntax do you use?

import React.Component from 'react'
import [ Component ] from 'react'
import Component from 'react'
import { Component } from 'react'

Q2. If a function component should always render the same way given the same props, what is a simple performance optimization available for it?

Wrap it in the React.memo higher-order component.
Implement the useReducer Hook.
Implement the useMemo Hook.
Implement the shouldComponentUpdate lifecycle method.

Q3. How do you fix the syntax error that results from running this code?

const person =(firstName, lastName) =>
{
  first: firstName,
  last: lastName
}
console.log(person("Jill", "Wilson"))

Wrap the object in parentheses.
Call the function from another file.
Add a return statement before the first curly brace.
Replace the object with an array.

Q4. If you see the following import in a file, what is being used for state management in the component?

import React, {useState} from 'react';

React Hooks
stateful components
math
class components

Q5. Using object literal enhancement, you can put values back into an object. When you log person to the console, what is the output?

const name = 'Rachel';
const age = 31;
const person = { name, age };
console.log(person);

{{name: "Rachel", age: 31}}
{name: "Rachel", age: 31}
{person: "Rachel", person: 31}}
{person: {name: "Rachel", age: 31}}

Q6. What is the testing library most often associated with React?

Mocha
Chai
Sinon
Jest

Q7. To get the first item from the array ("cooking") using array destructuring, how do you adjust this line?

const topics = ['cooking', 'art', 'history'];

const first = ["cooking", "art", "history"]
const [] = ["cooking", "art", "history"]
const [, first]["cooking", "art", "history"]
const [first] = ["cooking", "art", "history"]

Q8. How do you handle passing through the component tree without having to pass props down manually at every level?

React Send
React Pinpoint
React Router
React Context

Q9. What should the console read when the following code is run?

const [, , animal] = ['Horse', 'Mouse', 'Cat'];
console.log(animal);

Horse
Cat
Mouse
undefined

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

JSX Editor
ReactDOM
Browser Buddy
Babel

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

when you want to replace Redux
when you need to manage more complex state in an app
when you want to improve performance
when you want to break your production app

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

<Message {...props} />

any that have not changed
all of them
child props
any that have changed

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

<Route path="/:id" />

This is a route modal
This is a route parameter
This is a route splitter
This is a route link

14. If you created a component called Dish and rendered it to the DOM, what type of element would be rendered?

function Dish() {
  return <h1>Mac and Cheese</h1>;
}

ReactDOM.render(<Dish />, document.getElementById('root'));

div
section
component
h1

15. What does this React element look like given the following function? (Alternative: Given the following code, what does this React element look like?)

React.createElement('h1', null, "What's happening?");

<h1 props={null}>What's happening?</h1>
<h1>What's happening?</h1>
<h1 id="component">What's happening?</h1>
<h1 id="element">What's happening?</h1>

16. What property do you need to add to the Suspense component in order to display a spinner or loading state?

function MyComponent() {
  return (
    <Suspense>
      <div>
        <Message />
      </div>
    </Suspense>
  );
}

lazy
loading
fallback
spinner

17. What do you call the message wrapped in curly braces below?

const message = 'Hi there';
const element = <p>{message}</p>;

a JS function
a JS element
a JS expression
a JSX wrapper

18. What can you use to handle code splitting?

React.memo
React.split
React.lazy
React.fallback

19. When do you use `useLayoutEffect`?

to optimize for all devices
to complete the update
to change the layout of the screen
when you need the browser to paint before the effect runs

20. What is the difference between the click behaviors of these two buttons (assuming that this.handleClick is bound correctly)?

A. <button onClick={this.handleClick}>Click Me</button>
B. <button onClick={event => this.handleClick(event)}>Click Me</button>

Button A will not have access to the event object on click of the button.
Button B will not fire the handler this.handleClick successfully.
Button A will not fire the handler this.handleClick successfully.
There is no difference.

21. How do you destructure the properties that are sent to the Dish component?

function Dish(props) {
  return (
    <h1>
      {props.name} {props.cookingTime}
    </h1>
  );
}

function Dish([name, cookingTime]) { return <h1>{name} {cookingTime}</h1>; }
function Dish({name, cookingTime}) { return <h1>{name} {cookingTime}</h1>; }
function Dish(props) { return <h1>{name} {cookingTime}</h1>; }
function Dish(...props) { return <h1>{name} {cookingTime}</h1>; }

22. When might you use `React.PureComponent`?

when you do not want your component to have props
when you have sibling components that need to be compared
when you want a default implementation of shouldComponentUpdate()
when you do not want your component to have state

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

because you should never mutate state
because getDerivedStateFromProps() is an unsafe method to use
because you want to allow a component to update in response to changes in the props
because you want to allow data to flow back up to the parent

24. What is the children prop?

a property that adds child components to state
a property that lets you pass components as data to other components
a property that lets you set an array as a property
a property that lets you pass data to child elements

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

injectHTML
dangerouslySetInnerHTML
weirdSetInnerHTML
strangeHTML

26. Which of these terms commonly describe React applications?

declarative
integrated
closed
imperative

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

to put together physical file folders
to preprocess files
to load external data
to load the website into everyone's phone

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

virtual DOM
DOM
virtual elements
shadow DOM

29. You have written the following code but nothing is rendering. How do you fix this problem?

const Heading = () => {
  <h1>Hello!</h1>;
};

Add a render function.
Change the curly braces to parentheses or add a return statement before the h1 tag.
Move the h1 to another component.
Surround the h1 in a div.

Q30. To create a constant in JavaScript, which keyword do you use?

const
let
constant
var

Q31. What do you call a React component that catches JavaScript errors anywhere in the child component tree?

error bosses
error catchers
error helpers
error boundaries

Q32. In which lifecycle method do you make requests for data in a class component?

constructor
componentDidMount
componentWillReceiveProps
componentWillMount

Q33. React components are composed to create a user interface. How are components composed?

by putting them in the same file
by nesting components
with webpack
with code splitting

Q34. All React components must act like _ with respect to their props.

monads
pure functions
recursive functions
higher-order functions

Q35. Why might you use a ref?

to directly access the DOM node
to refer to another JS file
to call a function
to bind the function

Q36. What is `[e.target.id]` called in the following code snippet?

handleChange(e) {
  this.setState({ [e.target.id]: e.target.value })
}

a computed property name
a set value
a dynamic key
a JSX code string

Q37. What is the name of this component?

class Clock extends React.Component {
  render() {
    return <h1>Look at the time: {time}</h1>;
  }
}

Clock
It does not have a name prop.
React.Component
Component

Q38. What is sent to an `Array.map()` function?

a callback function that is called once for each element in the array
the name of another array to iterate over
the number of times you want to call the function
a string describing what the function should do

Q39. Why is it a good idea to pass a function to `setState` instead of an object?

It provides better encapsulation.
It makes sure that the object is not mutated.
It automatically updates a component.
setState is asynchronous and might result in out of sync values.

Explanation: Because this.props and this.state may be updated asynchronously, you should not rely on their values for calculating the next state. Read this article

Q40. What package contains the render() function that renders a React element tree to the DOM?

React
ReactDOM
Render
DOM

Q41. How do you set a default value for an uncontrolled form field?

Use the value property.
Use the defaultValue property.
Use the default property.
It assigns one automatically.

Q42. What do you need to change about this code to get it to run?

class clock extends React.Component {
  render() {
    return <h1>Look at the time: {this.props.time}</h1>;
  }
}

Add quotes around the return value
Remove this
Remove the render method
Capitalize clock

Explanation: In JSX, lower-case tag names are considered to be HTML tags. Read this article

Q43. Which Hook could be used to update the document's title?

useEffect(function updateTitle() { document.title = name + ' ' + lastname; });
useEffect(() => { title = name + ' ' + lastname; });
useEffect(function updateTitle() { name + ' ' + lastname; });
useEffect(function updateTitle() { title = name + ' ' + lastname; });

Q44. What can you use to wrap Component imports in order to load them lazily?

React.fallback
React.split
React.lazy
React.memo

Q45. How do you invoke setDone only when component mounts, using hooks?

function MyComponent(props) {
  const [done, setDone] = useState(false);

  return <h1>Done: {done}</h1>;
}

useEffect(() => { setDone(true); });
useEffect(() => { setDone(true); }, []);
useEffect(() => { setDone(true); }, [setDone]);
useEffect(() => { setDone(true); }, [done, setDone]);

Q46. Which of the following click event handlers will allow you to pass the name of the person to be hugged?

class Huggable extends React.Component {
  hug(id) {
    console.log("hugging " + id);
  }

  render() {
    let name = "kitteh";
    let button = // Missing Code
    return button;
  }
}

<button onClick={(name) => this.hug(name)}>Hug Button</button>
<button onClick={this.hug(e, name)}>Hug Button</button>
<button onClick={(e) => hug(e, name)}>Hug Button</button>
<button onClick={(e) => this.hug(name,e)}>Hug Button</button>

Q47. Currently, `handleClick` is being called instead of passed as a reference. How do you fix this?

<button onClick={this.handleClick()}>Click this</button>

<button onClick={this.handleClick.bind(handleClick)}>Click this</button>
<button onClick={handleClick()}>Click this</button>
<button onClick={this.handleClick}>Click this</button>
<button onclick={this.handleClick}>Click this</button>

Q48. Which answer best describes a function component?

A function component is the same as a class component.
A function component accepts a single props object and returns a React element.
A function component is the only way to create a component.
A function component is required to create a React component.

Q49. Which library does the `fetch()` function come from?

FetchJS
ReactDOM
No library. fetch() is supported by most browsers.
React

Q50. What will happen when this useEffect Hook is executed, assuming name is not already equal to John?

useEffect(() => {
  setName('John');
}, [name]);

It will cause an error immediately.
It will execute the code inside the function, but only after waiting to ensure that no other component is accessing the name variable.
It will update the value of name once and not run again until name is changed from the outside.
It will cause an infinite loop.

Q51. Which choice will not cause a React component to rerender?

if the component calls this.setState(...)
the value of one of the component's props changes
if the component calls this.forceUpdate()
one of the component's siblings rerenders

Q52. You have created a new method in a class component called handleClick, but it is not working. Which code is missing?

class Button extends React.Component{

  constructor(props) {
    super(props);
    // Missing line
  }

  handleClick() {...}
}

this.handleClick.bind(this);
props.bind(handleClick);
this.handleClick.bind();
this.handleClick = this.handleClick.bind(this);

Q53. React does not render two sibling elements unless they are wrapped in a fragment. Below is one way to render a fragment. What is the shorthand for this?

<React.Fragment>
  <h1>Our Staff</h1>
  <p>Our staff is available 9-5 to answer your questions</p>
</React.Fragment>

A

<...>
  <h1>Our Staff</h1>
  <p>Our staff is available 9-5 to answer your questions</p>
</...>

B

<//>
  <h1>Our Staff</h1>
  <p>Our staff is available 9-5 to answer your questions</p>
<///>

C

<>
  <h1>Our Staff</h1>
  <p>Our staff is available 9-5 to answer your questions</p>
</>

D

<Frag>
  <h1>Our Staff</h1>
  <p>Our staff is available 9-5 to answer your questions</p>
</Frag>

Q54. If you wanted to display the count state value in the component, what do you need to add to the curly braces in the `h1`?

class Ticker extends React.component {
  constructor(props) {
    super(props);
    this.state = { count: 0 };
  }
  render() {
    return <h1>{}</h1>;
  }
}

this.state.count
count
state
state.count

Q55. Per the following code, when is the Hello component displayed?

const greeting = isLoggedIn ? <Hello /> : null;

never
when isLoggedIn is true
when a user logs in
when the Hello function is called

Q56. In the following code block, what type is orderNumber?

ReactDOM.render(<Message orderNumber="16" />, document.getElementById('root'));

string
boolean
object
number

Q57. You have added a style property to the `h1` but there is an unexpected token error when it runs. How do you fix this?

const element = <h1 style={ backgroundColor: "blue" }>Hi</h1>;

const element = <h1 style="backgroundColor: "blue""}>Hi</h1>;
const element = <h1 style={{backgroundColor: "blue"}}>Hi</h1>;
const element = <h1 style={blue}>Hi</h1>;
const element = <h1 style="blue">Hi</h1>;

Q58. Which function is used to update state variables in a React class component?

replaceState
refreshState
updateState
setState

Q59. Consider the following component. What is the default color for the star?

const Star = ({ selected = false }) => <Icon color={selected ? 'red' : 'grey'} />;

black
red
grey
white

Q60.Which library does the fetch() function come from?

FetchJS
ReactDOM
No library. fetch() is supported by most browsers.
React

Q61.What is the difference between the click behaviors of these two buttons(assuming that this.handleClick is bound correctly)

  A. <button onClick=this.handleClick>Click Me</button>
  B. <button onClick={event => this.handleClick(event)}>Click Me</button>

Button A will not have access to the event object on click of the button
Button A will not fire the handler this.handleClick successfully
There is no difference
Button B will not fire the handler this.handleClick successfully

Q62.What will happen when this useEffect Hook is executed, assuming name is not already equal to John?

useEffect(() => {
  setName('John');
}, [name]);

It will cause an error immediately.
It will execute the code inside the function, but only after waiting to ensure that no other component is accessing the name variable.
It will update the value of name once and not run again until name is changed from the outside.
It will cause an infinite loop.

Q63. How would you add to this code, from React Router, to display a component called About?

<Route path="/:id" />

A

<Route path="/:id">
  {' '}
  <About />
</Route>

B

<Route path="/tid" about={Component} />

C

<Route path="/:id" route={About} />

D

<Route>
  <About path="/:id" />
</Route>

Q64. Which class-based component is equivalent to this function component?

const Greeting ({ name }) > <h1>Hello {name}!</h1>;

A

class Greeting extends React.Component {
  constructor() {
    return <h1>Hello {this.props.name}!</h1>;
  }
}

B

class Greeting extends React.Component {
  <h1>Hello {this.props.name}!</h1>;
}

C

class Greeting extends React.Component {
  render() {
    return <h1>Hello {this.props.name}!</h1>;
  }
}

D

class Greeting extends React.Component {
  render({ name }) {
    return <h1>Hello {name}!</h1>;
  }
}

Q65. Give the code below, what does the second argument that is sent to the render function describe?

ReactDOM.render(
  <h1>Hi<h1>,
    document.getElementById('root')
)

where the React element should be added to the DOM
where to call the function
where the root component is
where to create a new JavaScript file

Q66. Why should you use React Router's Link component instead of a basic `<a>` tag in React?

The link component allows the user to use the browser's Back button.
There is no difference--the Link component is just another name for the <a> tag.
The <a> tag will cause an error when used in React.
The <a> tag triggers a full page reload, while the Link component does not.

Q67. What is the first argument, `x`, that is sent to the `createElement` function?

React.createElement(x, y, z);

the element that should be created
the order in which this element should be placed on the page
the properties of the element
data that should be displayed in the element

Q68. Which class-based lifecycle method would be called at the same time as this effect Hook?

useEffect(() => {
  // do things
}, []);

componentWillUnmount
componentDidMount
render
componentDidUpdate

Q69. Given the code below, what does the second argument that is sent to the render function describe?

ReactDOM.render(<h1>Hi</h1>, document.getElementById('root'));

where the React element should be added to the DOM
where to call the function
where the root component is
where to create a new JavaScript file

Q70. What is the first argument, x, that is sent to the `createElement` function?

React.createElement(x,y,z);

the element that should be created
the order in which this element should be placed on the page
the properties of the element
data that should be displayed in the element.

Q71. What is the name of this component?

class Comp extends React.Component {
  render() {
    return <h1>Look at the time: {time}</h1>;
  }
}

Comp
h1
React.Component
Component

This question might be an updated version of Q37.

Q72. When using a portal, what is the first argument?

ReactDOM.createPortal(x, y);

the current state
the element to render
the App component
the page

Explanation: From official docs: Portals

Q73. What is `setCount`?

const [count, setCount] = useState(0);

the initial state value
a variable
a state object
a function to update the state

Reference: From official docs: Hooks-State

Q74. What is the use of map function below?

const database = [user1:{},user2:{},user3:{}];
database.map((user)=><h1>user.data</h1>);

gives a map of all the entries in database
returns a heading tag for every entry in the database containing it's data
returns one heading tag for all the entries in database
checks which entry in the database is suitable for heading tag

Q75. Describe what is happening in this code?

const { name: firstName } = person;

It is creating a new object that contains the same name property as the person object.
It is assigning the value of the person object's firstName property to a constant called name.
It is retrieving the value of person.name.firstName.
It is assigning the value of the person object's name property to a constant called firstName.

Q76. What is wrong with this code?

const MyComponent = ({ names }) => (
  <h1>Hello</h1>
  <p>Hello again</p>
);

React components cannot be defined using functions.
React does not allow components to return more than one element.
The component needs to use the return keyword.
String literals must be surrounded by quotes.

Q77. When using a portal, what is the second argument?

ReactDOM.createPortal(x, y);

the App component
the page
the current state
the DOM element that exists outside of the parent component

Q78. Given this code, what will be printed in the `<h1>` tag?

const MyComponent = ({ children }) => (
  <h1>{children.length}</h1>
);
...
<MyComponent>
<p>Hello</p>
<p>Goodbye</p>
</MyComponent>

It will produce an error saying "cannot read property "length" of undefined."
1
undefined
2

Q79. What is this pattern called?

const [count, setCount] = useState(0);

object destructuring
array destructuring
spread operating
code pushing

Q80. What is the first file loaded by the browser in a basic React project?

src/App.js
src/index.js
public/manifest.json
public/index.html

Q81. The code below is rendering nothing, and there is an error that says "ReactDOM is not defined." How do you fix this issue?

import React from 'react';
import { render } from 'react-dom';

const element = <h1>Hi</h1>;

ReactDOM.render(element, document.getElementById('root'));

render(element, document.getElementById("root"));
ReactDOM(element, document.getElementById("root"));
renderDOM(element, document.getElementById("root"));
DOM(element, document.getElementById("root"));

Q82. In this component, how do you display whether the user was logged in or not?

render() {
  const isLoggedIn = this.state.isLoggedIn;
  return (
    <div>
      The user is:
    </div>
  );
}

The user is loggedIn ? logged in : not logged in.
Write a function to check the login status.
The user is {isLoggedIn = "no"}.
The user is {isLoggedIn ? "logged in." : "not logged in"}.

Q83. 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?

Pass the name of each item as its key.
Add a key prop with the same value to each item the list.
Clear the console warnings.
When iterating over the list items, add a unique property to each list item.

Q84. How would you generate the boilerplate code for a new app that you are building to collect underpants?

npm create-react-app collect-underpants
npx start-app collect-underpants
react new collect-underpants
npx create-react-app collect-underpants

Source: React Docs

Q85. Add the code that will fire the photon torpedoes when the button is clicked.

class StarTrekkin extends React.Component {
  firePhotonTorpedoes(e) {
    console.log('pew pew');
  }
  render() {
    return; // Missing code
  }
}

<button onClick={firePhotonTorpedoes()}>Pew Pew</button>
<button onClick={firePhotonTorpedoes}>Pew Pew</button>
<button onClick={this.firePhotonTorpedoes()}>Pew Pew</button>
<button onClick={this.firePhotonTorpedoes}>Pew Pew</button>

Source: React Docs

Q86. What is the process of deciding whether an update is necessary?

shadow DOM
fiber
reconciliation
setting state

Q87. React is an open-source project but is maintained by which company?

Intuit
Twitter
Facebook
Snapchat

Q88. What command can you use to generate a React project?

react-starter
create-react-app
react-gen
react-start

Q89. What is the browser extension called that React developers use to debug applications?

React Developer Tools
React Tooling Add-on
React Codewatch
React Debug

Q90. Which tool is not part of Create React App?

React
jQuery
webpack
ReactDOM

Q91. What is the JavaScript syntax extension that is commonly used to create React elements?

HTML
JavaScriptX
JSX
React JavaScript

Q92. How might you check property types without using Flow or TypeScript?

Check Manually.
Use prop-helper.
use prop-types.
user checker-types.

Q93. How do you add an id of heading to the following h1 element?

let dish = <h1>Mac and Cheese</h1>;

let dish = <h1 id={heading}>Mac and Cheese</h1>;
let dish = <h1 id="heading">Mac and Cheese</h1>;
let dish = <h1 id:"heading">Mac and Cheese</h1>;
let dish = <h1 class="heading">Mac and Cheese</h1>;

Q94. What value of button will allow you to pass the name of the person to be hugged?

class Huggable extends React.Component {
  hug(id) {
    console.log("hugging " + id);
  }
  render() {
    let name = "kitten";
    let button = // Missing code
    return button;
  }
}

<button onClick={(name) => this.hug(name)}>Hug Button</button>;
<button onClick={this.hug(e, name)}>Hug Button</button>;
<button onClick={(e) => hug(name, e)}>Hug Button</button>;
<button onClick={(e) => this.hug(name, e)}>Hug Button</button>;

Explanation: This question test knowledge of react class components. You need to use this in order to call methods declared inside class components.

Q95. What syntax do you use to create a component in React?

a generator
a function or a class
a service worker
a tag

React Components are like functions that return HTML elements. Components are independent and reusable bits of code. They serve the same purpose as JavaScript functions, but work in isolation and return HTML. Components come in two types, Class components and Function components. (Source)

Q96. You want to disable a button so that it does not emit any events onClick. Which prop do you use to acomplish this?

onBlur
onPress
defaultValue
disabled

Q97. In this function, which is the best way to describe the Dish component?

function Dish() {
  return (
    <>
      <Ingredient />
      <Ingredient />
    </>
  );
}

child component
parent component
nested component
sibling component

Q98. When does the componentDidMount function fire?

right after the component is added to the DOM
before the component is added to the DOM
right after the component is updated
right after an API call

Q99. What might you use webpack for?

to fetch remote dependencies used by your app
to split your app into smaller chunks that can be more easily loaded by the browser
to format your code so that it is more readable
to ensure your app is not vulnerable to code injection

Q100. When using the React Developer Tools Chrome extension, what does it mean if the React icon is red?

You are using the development build of React.
You are using the production build of React.
You are using webpack.
You are using Create React App.

Reference

Q101. How would you modify the constructor to fix this erroe: "ReferenceError: Must call super constructor in derived class before accessing 'this'..."?

class TransIsBeautiful extends React.Component {
  constructor(props){
  // Missing line
  console.log(this) ;
  }
  ...
}

render(props);
super(props);
super(this);
this.super();

Q102. Which language can you not use with React?

Swift.
JSX.
Javascipt.
TypeScript.

Q103. This code is part of an app that collects Pokemon. How would you print the list of the ones collected so far?

constructor(props) {
    super(props);
    this.state = {
        pokeDex: []
    };
}

console.log(props.pokeDex);
console.log(this.props.pokeDex);
console.log(pokeDex);
console.log(this.state.pokeDex);