Creating first React app - Quick Revision & Summary

Recall & Review

beginner

What command do you use to create a new React app using the official tool?

You use npx create-react-app my-app to create a new React app named 'my-app'.

beginner

What is the main file where React components start rendering in a new React app?

The main file is src/index.js. It renders the root React component into the HTML page.

beginner

Which React hook is commonly used to add state to a functional component?

The useState hook lets you add state to functional components.

beginner

How do you start the React development server to see your app in the browser?

Run npm start inside your React app folder to start the development server.

beginner

What file contains the HTML structure that React renders into?

The public/index.html file contains the HTML where React inserts the app.

Which command creates a new React app?

A npm react create

B npm install react-app

C react new app

D npx create-react-app my-app

The correct command is npx create-react-app my-app to create a new React app.

Where does React render the app in the HTML file?

A <div id="root"></div>

B <body>

C <header>

D <main>

React renders the app inside the <div id="root"></div> element.

Which file is the main entry point for React components?

A src/index.js

B public/index.html

C src/App.css

D package.json

The main entry point is src/index.js, where React starts rendering.

What command starts the React development server?

A npm run dev

B npm start

C npm build

D npm serve

Use npm start to launch the React development server.

Which hook is used to add state in a React functional component?

A useRef

B useEffect

C useState

D useContext

useState is the hook to add state inside functional components.

Describe the steps to create and run your first React app from scratch.

Think about commands and what happens after creating the app.

Explain the role of the 'src/index.js' and 'public/index.html' files in a React app.

One is HTML, the other is JavaScript starting point.

Practice

1. What is the main purpose of the ReactDOM.createRoot method in a React app?

easy

A. It creates a new React component.

B. It tells React where to put the app in the HTML page.

C. It styles the React app automatically.

D. It fetches data from the server.

Solution

1. Step 1: Understand ReactDOM.createRoot usage

   This method is used to select the HTML element where the React app will appear, usually a div with id 'root'.

2. Step 2: Identify its role in rendering

   It creates a root for React to manage the app's UI inside that HTML element.

3. Final Answer:

   It tells React where to put the app in the HTML page. -> Option B

4. Quick Check:

   ReactDOM.createRoot sets the app's HTML container [OK]

Hint: ReactDOM.createRoot links React to HTML element [OK]

Common Mistakes:

• Thinking it creates components
• Confusing it with styling or data fetching
• Assuming it runs app logic

2. Which of the following is the correct way to define a simple React functional component named App?

easy

A. class App extends React.Component { render() { return 'Hello'; } }

B. const App = Hello;

C. function App() { return Hello; }

D. function App { return Hello; }

Solution

1. Step 1: Check function syntax for React component

   A React functional component is a function that returns JSX, so it needs parentheses after the name.

2. Step 2: Validate JSX return and syntax

   function App() { return
   Hello; } correctly defines a function named App that returns JSX inside a div.

3. Final Answer:

   function App() { return
   Hello
   ; } -> Option C

4. Quick Check:

   Functional component = function with () and JSX return [OK]

Hint: React components are functions returning JSX with () [OK]

Common Mistakes:

• Missing parentheses after function name
• Assigning JSX directly to a variable without function
• Using class syntax incorrectly for functional components

3. What will be displayed on the webpage after running this React code?

import React from 'react';
import ReactDOM from 'react-dom/client';

function App() {
  return 
Welcome to React!
;
}

const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<App />);

medium

A. undefined

B. App

C. Nothing, there is an error

D. Welcome to React!

Solution

1. Step 1: Understand what the App component returns

   The App function returns an <h1> element with the text 'Welcome to React!'.

2. Step 2: Check rendering process

   The root renders the App component inside the HTML element with id 'root', so the heading will appear on the page.

3. Final Answer:

   Welcome to React! -> Option D

4. Quick Check:

   App returns <h1>Welcome to React!</h1> [OK]

Hint: Component returns JSX shown on page [OK]

Common Mistakes:

• Thinking component name shows instead of its return
• Assuming error without checking code carefully
• Confusing JSX tags with plain text

4. Identify the error in this React app code:

import React from 'react';
import ReactDOM from 'react-dom/client';

function App() {
  return <h1>Hello World</h1>;
}

const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(App);

medium

A. App should be called as in render, not just App.

B. Missing import of ReactDOM.

C. Function App must return a string, not JSX.

D. document.getElementById('root') is incorrect.

Solution

1. Step 1: Check how render is called

   React expects a JSX element or component instance in render, so is correct, not just App.

2. Step 2: Understand difference between component and element

   App is a function, is JSX that creates an element from that function.

3. Final Answer:

   App should be called as <App /> in render, not just App. -> Option A

4. Quick Check:

   render needs JSX element like <App /> [OK]

Hint: Use to render component, not App alone [OK]

Common Mistakes:

• Passing function name instead of JSX element to render
• Confusing component function with element instance
• Ignoring JSX syntax rules

5. You want to create a React app that shows a greeting message inside a div with a class container. Which code correctly creates and renders this app?

hard

A. function App() { return Hello!; }
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render();

B. function App() { return Hello!; }
ReactDOM.render(, document.getElementById('root'));

C. const App = () => Hello!;
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(App);

D. function App() { return Hello!; }
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(App);

Solution

1. Step 1: Use correct JSX attribute for class

   In React, use className instead of class for CSS classes.

2. Step 2: Render the component as JSX element

   Use root.render(<App />) to render the component, not just the function name.

3. Final Answer:

   function App() { return <div className="container">Hello!</div>; } const root = ReactDOM.createRoot(document.getElementById('root')); root.render(<App />); -> Option A

4. Quick Check:

   Use className and render JSX element [OK]

Hint: Use className and render with [OK]

Common Mistakes:

• Using class instead of className in JSX
• Passing function name instead of JSX element to render
• Using old ReactDOM.render instead of createRoot