Creating first React app

Introduction

React helps you build interactive web pages easily. Creating your first React app shows you how to start using React step-by-step.

You want to build a website that updates parts of the page without reloading.

You want to learn how to make reusable pieces of a webpage called components.

You want to practice writing JavaScript that works with HTML in a simple way.

You want to create a project that can grow bigger with many interactive parts.

Syntax

React

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

function App() {
  return (
    <div>
      <h1>Hello, React!</h1>
    </div>
  );
}

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

Use function App() to create a component that returns HTML-like code called JSX.

Use ReactDOM.createRoot and root.render to show your component on the webpage.

Examples

A simple component that shows a heading.

React

function App() {
  return <h1>Welcome to React!</h1>;
}

This code finds the webpage spot with id 'root' and puts your App component there.

React

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

Sample Program

This React app shows a heading and a paragraph on the page. It uses a component called App and renders it inside the element with id root.

React

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

function App() {
  return (
    <main>
      <h1>Hello, React!</h1>
      <p>This is your first React app.</p>
    </main>
  );
}

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

Important Notes

Make sure your HTML file has a <div id="root"></div> where React can put your app.

Use a tool like Create React App or Vite to set up your project easily.

JSX looks like HTML but it is JavaScript. Always return one parent element from your component.

Summary

React apps are built from components that return JSX.

You render your main component inside a webpage element with an id like 'root'.

Creating your first React app helps you start building interactive web pages.

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