Creating first React app - Performance Optimization Steps

Performance: Creating first React app

MEDIUM IMPACT

This affects the initial page load speed and time to interactive by controlling how React and its dependencies load and render the app.

Setting up the first React app with optimal loading

React

import React from 'react';
import { createRoot } from 'react-dom/client';

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

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

Using createRoot from 'react-dom/client' enables concurrent features and better startup performance; also supports future optimizations.

📈 Performance GainReduces blocking time by 20-50ms; enables smoother initial rendering.

Setting up the first React app with optimal loading

React

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

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

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

Using ReactDOM.render from 'react-dom' blocks rendering until React and ReactDOM fully load, increasing initial load time.

📉 Performance CostBlocks rendering for 200-400ms on slow networks; large initial bundle (~100kb) without code splitting.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
ReactDOM.render (legacy)	Single root node	1 reflow	Low paint cost	[!] OK
createRoot.render (modern)	Single root node	1 reflow	Low paint cost	[OK] Good

Rendering Pipeline

The React app loading flows through parsing JS, executing React code, building virtual DOM, then updating the real DOM. Initial bundle size and script loading affect how fast this pipeline starts.

→Script Parsing

→JS Execution

→Virtual DOM Creation

→DOM Update

⚠️ BottleneckJS Execution and DOM Update are most expensive due to large bundle and synchronous rendering calls.

Core Web Vital Affected

LCP

This affects the initial page load speed and time to interactive by controlling how React and its dependencies load and render the app.

Optimization Tips

1Use createRoot from react-dom/client instead of ReactDOM.render for better startup performance.

2Keep initial JavaScript bundle small to reduce blocking time and improve LCP.

3Use code splitting and lazy loading to defer non-critical code and speed up first paint.

Performance Quiz - 3 Questions

Which React API improves initial rendering performance in a new React app?

A createRoot from react-dom/client

B ReactDOM.render from react-dom

C React.createElement

D React.useState

DevTools: Performance

How to check: Open DevTools > Performance tab, record page load, and look for long scripting or blocking times related to React scripts.

What to look for: Look for long tasks during JS execution and delayed first contentful paint indicating slow React app startup.

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