What we know:

A top-level App component returns <Button /> from its render() method.

Question:

What is the relationship between <Button /> and this in that Button's render()?

Answer:

<Button> is a React "element".
If you log it, you will see a plain object like { type: Button, props: {} }.

The element does not represent anything on the screen at that point.
It is a description of what App wants to be rendered.

At some point, React will look at that description and think: "Hmm, there was no Button here but now there should be. So I'll create a Button instance."

This Button instance created by React is this value in the render() and lifecycle methods. It's only useful for calling setState() or reading the props and state.

If App gets re-rendered and React sees a <Button /> in its output again, it will think: "Hmm, I already have a Button instance exactly at the same spot. Rather than create a new one, I will just update props on the existing one and re-render it."

Let's recap.

<Button /> is an element, a description of what should be rendered. this inside the Button is the actual instance React created based on that description.

App can return a different <Button /> element every time but as long as its key doesn't change, and it is still a <Button /> and not a <Door />, React will keep using the same instance.

Question:

Does rendering <Button><Icon /></Button> guarantee that an Icon mounts?

It doesn't. Ultimately it's always up to the component to decide what to do with its children. For example, the Button implementation could completely ignore them and render something else:

function Button(props) {
    return <h1>I render whatever I want</h1>;
}

Interestingly, it could also render children multiple times:

function Button(props) {
    return (
        <div>
            I like to repeat things.
            {props.children}
            {props.children}
            {props.children}
        </div>
    );
}

Ultimately, children is not a special prop in any way except for JSX sugar syntax. <Button><Icon /></Button> is technically the same as <Button children={<Icon />} />, and it's up to the component how to treat its input props.

Question:

Can the App change anything in the Button output? What and how?

Answer:

There have been a few fun answers in the quiz comments so I will direct you to them. Indeed, since JavaScript doesn't provide any guarantees, technically you can hijack Button before React gets a chance to render it.

However, normally there is no way for a parent component to control the child output except by two mechanisms: passing props and providing context. You probably already know about props, and I won't talk about the context because it's an experimental API and has a few pitfalls. Don't use context in apps unless you know its pitfalls well.

Of course, if you get a DOM node for the component via refs, you can do anything you want with the DOM nodes of other components, but it will likely mess up React.

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.

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 answer best describes a function component?(Not sure answer)

• 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.

Q61.Which library does the fetch() function come from?

• FetchJS
• ReactDOM
• [✅] No library. fetch() is supported by most browsers.
• React

Q62.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

Q63.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.

Q64. 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>

Q65. 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>;
    }
}

Q66. 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

Q67. 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.

Q68. 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

Q69. Which class-based lifecycle method would be called at the same time as this effect Hook?

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

• componentWillUnmount
• [✅] componentDidMount
• render
• componentDidUpdate

Q70. 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

Q71. 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.

Q72. 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.

Q73. 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

Q74. 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

Q75. 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

Q76. 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.

Q77. 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.

Q78. 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

Q79. Given this code, what will be printed in the

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

Q80. What is this pattern called?

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

• object destructuring
• [✅] array destructuring
• spread operating
• code pushing