Quick Answer

ReactJS is built on JavaScript. Every React application is written in JavaScript (or TypeScript, a typed superset of JavaScript). React also uses JSX a syntax extension that lets you write HTML-like code inside JavaScript along with HTML for structure and CSS for styling. However, JavaScript is the only true programming language powering React. JSX, HTML, and CSS are supporting technologies that work alongside it.

Here’s the complete breakdown of every language and technology involved in React development.

JavaScript: The Core Language of React

JavaScript is the foundation of ReactJS. React is a JavaScript library every component you build, every state update you trigger, and every event you handle is JavaScript executing in the browser.

This isn’t a coincidence. When Jordan Walke created React at Facebook in 2011, he built it as a JavaScript library because JavaScript was (and remains) the only programming language that runs natively in web browsers. Every browser ships with a JavaScript engine V8 in Chrome, SpiderMonkey in Firefox, JavaScriptCore in Safari making JavaScript the universal language of the web.

Why JavaScript Specifically?

React uses JavaScript for several strategic reasons:

1. Universal Browser Support

JavaScript runs in every browser without plugins, compilers, or additional setup. When a user visits a React-powered website, the browser’s JavaScript engine executes the code directly. No other programming language has this universal browser support.

2. Event-Driven Architecture

Web applications need to respond to user actions clicks, keystrokes, scrolls, form submissions. JavaScript’s event-driven model handles this naturally:

javascript

// JavaScript event handling in React
function SearchBar() {
  const [query, setQuery] = useState('');

  const handleChange = (event) => {
    setQuery(event.target.value);    // Captures every keystroke
  };

  const handleSubmit = (event) => {
    event.preventDefault();           // Prevents page reload
    searchProducts(query);            // Triggers search
  };

  return (
    <form onSubmit={handleSubmit}>
      <input 
        type="text" 
        value={query} 
        onChange={handleChange}
        placeholder="Search products..."
      />
      <button type="submit">Search</button>
    </form>
  );
}

Every interaction in this component typing, submitting, preventing default behavior is handled by JavaScript.

3. Asynchronous Data Fetching

Modern web apps pull data from APIs. JavaScript’s async/await and fetch API make this seamless:

javascript

// Fetching data from an API in React
function UserProfile({ userId }) {
  const [user, setUser] = useState(null);
  const [loading, setLoading] = useState(true);

  useEffect(() => {
    const fetchUser = async () => {
      try {
        const response = await fetch(`https://api.example.com/users/${userId}`);
        const data = await response.json();
        setUser(data);
      } catch (error) {
        console.error('Failed to fetch user:', error);
      } finally {
        setLoading(false);
      }
    };
    fetchUser();
  }, [userId]);

  if (loading) return <p>Loading...</p>;
  return <h1>{user.name}</h1>;
}

This pattern fetching data, updating state, re-rendering the UI is the backbone of every React application, and it’s all JavaScript.

4. Dynamic DOM Manipulation

JavaScript can read, modify, and create HTML elements programmatically. React abstracts this through its Virtual DOM, but underneath, JavaScript is doing the actual work of updating what users see on screen.

Understanding JavaScript’s role clarifies a common misconception React itself is not a programming language, it’s a JavaScript library that provides tools for building UIs more efficiently.

ES6+ Features Essential for React

Modern React development relies heavily on ES6+ (ECMAScript 2015 and later) features. If you’re writing React in 2026, you’re using these daily:

1. Arrow Functions

Arrow functions are the standard for React event handlers, callbacks, and component definitions:

javascript

// Traditional function
function handleClick() {
  console.log('Clicked!');
}

// Arrow function (React standard)
const handleClick = () => {
  console.log('Clicked!');
};

// Inline arrow function in JSX
<button onClick={() => setCount(count + 1)}>Increment</button>

Why React uses them: Shorter syntax, lexical this binding (avoids common bugs in class components), and cleaner inline callbacks.

2. Destructuring

Destructuring extracts values from objects and arrays. React uses it everywhere:

javascript

// Object destructuring — React props
function ProductCard({ name, price, image, onAddToCart }) {
  return (
    <div className="product">
      <img src={image} alt={name} />
      <h3>{name}</h3>
      <p>${price.toFixed(2)}</p>
      <button onClick={onAddToCart}>Add to Cart</button>
    </div>
  );
}

// Array destructuring — React hooks
const [isVisible, setIsVisible] = useState(false);
const [users, setUsers] = useState([]);
const [error, setError] = useState(null);

3. Spread and Rest Operators

javascript

// Spread — passing props
const userProps = { name: 'Alice', age: 28, email: 'alice@example.com' };
<UserCard {...userProps} />

// Spread — immutable state updates
const [items, setItems] = useState(['Apple', 'Banana']);
setItems([...items, 'Cherry']);  // Adds without mutating

// Rest — collecting remaining props
function Button({ variant, size, ...rest }) {
  return <button className={`btn-${variant} btn-${size}`} {...rest} />;
}

4. Template Literals

javascript

// Dynamic strings in React
const greeting = `Hello, ${user.name}!`;
const className = `card ${isActive ? 'card-active' : 'card-inactive'}`;
const apiUrl = `https://api.example.com/users/${userId}/posts`;

5. Modules (import/export)

Every React application is organized into modules:

javascript

// Named exports
export function formatPrice(price) {
  return `$${price.toFixed(2)}`;
}
export const TAX_RATE = 0.08;

// Default export (one per file)
export default function App() {
  return <h1>My Application</h1>;
}

// Importing
import App from './App';
import { formatPrice, TAX_RATE } from './utils';
import React, { useState, useEffect, useContext } from 'react';

6. Array Methods (map, filter, reduce)

These are critical for rendering lists and transforming data in React:

javascript

function ProductList({ products }) {
  return (
    <div>
      {/* .filter() — conditional rendering */}
      {products
        .filter(product => product.inStock)

        {/* .map() — rendering lists */}
        .map(product => (
          <ProductCard 
            key={product.id}
            name={product.name} 
            price={product.price} 
          />
        ))
      }

      {/* .reduce() — calculating totals */}
      <p>Total: ${products.reduce((sum, p) => sum + p.price, 0).toFixed(2)}</p>
    </div>
  );
}

7. Promises and Async/Await

javascript

// Modern async pattern in React
useEffect(() => {
  const loadData = async () => {
    const [usersRes, postsRes] = await Promise.all([
      fetch('/api/users'),
      fetch('/api/posts')
    ]);
    const users = await usersRes.json();
    const posts = await postsRes.json();
    setUsers(users);
    setPosts(posts);
  };
  loadData();
}, []);

8. Optional Chaining and Nullish Coalescing

javascript

// Safe property access (avoids "Cannot read property of undefined")
const userName = user?.profile?.name ?? 'Anonymous';
const avatarUrl = user?.avatar?.url ?? '/default-avatar.png';

// In JSX
<p>{order?.items?.length ?? 0} items in cart</p>

Complete ES6+ Features Used in React

Feature	React Usage	Frequency
Arrow functions	Event handlers, callbacks, components	Every file
Destructuring	Props, hooks, API responses	Every component
Spread/Rest	Props forwarding, state updates	Very frequent
Template literals	Dynamic strings, class names, URLs	Frequent
import/export	Module organization	Every file
map/filter/reduce	List rendering, data transformation	Most components
async/await	API calls, data fetching	Data-driven components
Optional chaining	Safe property access	Frequent
Ternary operator	Conditional rendering	Every component
const/let	Variable declarations	Every file

JSX: React’s Signature Syntax

JSX (JavaScript XML) is a syntax extension that lets you write HTML-like code inside JavaScript. It’s React’s most distinctive feature and the primary reason people sometimes confuse React with a programming language.

What JSX Looks Like

jsx

function Dashboard({ user, notifications }) {
  return (
    <div className="dashboard">
      <header>
        <h1>Welcome back, {user.name}</h1>
        <span className="badge">{notifications.length}</span>
      </header>

      {notifications.length > 0 ? (
        <ul className="notification-list">
          {notifications.map(notif => (
            <li key={notif.id} className={notif.read ? 'read' : 'unread'}>
              {notif.message}
            </li>
          ))}
        </ul>
      ) : (
        <p>No new notifications</p>
      )}
    </div>
  );
}

What JSX Compiles To

JSX isn’t understood by browsers. Tools like Babel transform it into standard JavaScript before execution:

javascript

// The JSX above becomes this JavaScript:
function Dashboard({ user, notifications }) {
  return React.createElement(
    'div',
    { className: 'dashboard' },
    React.createElement(
      'header',
      null,
      React.createElement('h1', null, 'Welcome back, ', user.name),
      React.createElement('span', { className: 'badge' }, notifications.length)
    ),
    // ... and so on
  );
}

JSX is syntactic sugar — it makes React.createElement() calls readable. You could write React entirely without JSX, but nobody does because the code becomes extremely verbose.

JSX Rules You Must Know

Rule	Correct	Wrong
Single root element	<div>...</div> or <>...</>	Multiple sibling elements without wrapper
Close all tags	<img />, <br />, <input />	<img>, <br>, <input>
Use className	className="header"	class="header"
Use camelCase	onClick, onChange, htmlFor	onclick, onchange, for
Expressions in {}	{user.name}, {2 + 2}	Direct JS without braces
No if/else in JSX	Use ternary: {x ? A : B}	{if (x) { A } else { B }}

JSX vs. HTML: Key Differences

{/* JSX */}
<div className="container">
  <label htmlFor="email">Email</label>
  <input 
    id="email" 
    type="text" 
    onChange={handleChange}
    style={{ backgroundColor: '#f0f0f0', fontSize: '16px' }}
  />
  <img src={logoUrl} alt="Company logo" />
</div>

html

<!-- HTML -->
<div class="container">
  <label for="email">Email</label>
  <input 
    id="email" 
    type="text" 
    onchange="handleChange()"
    style="background-color: #f0f0f0; font-size: 16px;"
  >
  <img src="logo.png" alt="Company logo">
</div>

Key differences:

• class → className (because class is a reserved word in JavaScript)
• for → htmlFor (same reason)
• onclick → onClick (camelCase for all event handlers)
• style="" → style={{}} (JavaScript object, not CSS string)
• All tags must be closed (self-closing: <img />, <input />)

TypeScript: The Typed Alternative

TypeScript is a superset of JavaScript that adds static type checking. It’s become the preferred choice for large-scale React applications.

What TypeScript Adds to React

typescript

// TypeScript React component with type safety
interface User {
  id: number;
  name: string;
  email: string;
  role: 'admin' | 'user' | 'editor';
}

interface UserCardProps {
  user: User;
  onEdit: (id: number) => void;
  showEmail?: boolean;  // Optional prop
}

function UserCard({ user, onEdit, showEmail = true }: UserCardProps) {
  return (
    <div className="user-card">
      <h3>{user.name}</h3>
      {showEmail && <p>{user.email}</p>}
      <span className={`role-badge role-${user.role}`}>{user.role}</span>
      <button onClick={() => onEdit(user.id)}>Edit</button>
    </div>
  );
}

Why TypeScript is Popular for React

Benefit	How It Helps
Catches errors early	Type mismatches caught during development, not in production
Better autocomplete	IDEs suggest correct props, methods, and values
Self-documenting code	Interfaces describe component APIs clearly
Safer refactoring	Changing a prop name highlights every usage
Team scalability	Easier for new developers to understand codebases

TypeScript Adoption in React

• 78% of React developers use TypeScript in production (State of JS 2024)
• Next.js, Remix, and Vite all default to TypeScript setup
• All major React libraries (React Router, Redux, TanStack Query) ship TypeScript definitions
• Create React App and Vite templates offer TypeScript out of the box

Setting Up TypeScript with React

bash

# New React project with TypeScript (Vite)
npm create vite@latest my-app -- --template react-ts

# New Next.js project with TypeScript
npx create-next-app@latest my-app --typescript

# Add TypeScript to existing React project
npm install typescript @types/react @types/react-dom

HTML and CSS: The Supporting Technologies

While JavaScript is React’s language, HTML and CSS play essential supporting roles.

HTML in React

React doesn’t use HTML files directly. Instead, JSX provides HTML-like syntax that React compiles into actual DOM elements. However, understanding HTML is critical because:

• JSX mirrors HTML structure (tags, attributes, nesting)
• Semantic HTML improves accessibility and SEO
• You need to know HTML elements to write correct JSX

jsx

{/* JSX uses HTML elements — but it's still JavaScript */}
function ArticlePage({ title, content, author }) {
  return (
    <article>
      <header>
        <h1>{title}</h1>
        <address>By {author}</address>
      </header>
      <main dangerouslySetInnerHTML={{ __html: content }} />
      <footer>
        <nav aria-label="Article navigation">
          <a href="/previous">Previous</a>
          <a href="/next">Next</a>
        </nav>
      </footer>
    </article>
  );
}

CSS in React

React applications use CSS for styling, but with several approach options:

1. Regular CSS Files

css

/* styles.css */
.product-card {
  border: 1px solid #e0e0e0;
  border-radius: 8px;
  padding: 16px;
}

jsx

import './styles.css';
function ProductCard() {
  return <div className="product-card">...</div>;
}

2. CSS Modules (Scoped Styles)

jsx

import styles from './ProductCard.module.css';
function ProductCard() {
  return <div className={styles.card}>...</div>;
}

3. CSS-in-JS (Styled Components)

jsx

import styled from 'styled-components';

const Card = styled.div`
  border: 1px solid #e0e0e0;
  border-radius: 8px;
  padding: 16px;
  
  &:hover {
    box-shadow: 0 4px 12px rgba(0,0,0,0.1);
  }
`;

function ProductCard() {
  return <Card>...</Card>;
}

4. Tailwind CSS (Utility Classes)

jsx

function ProductCard() {
  return (
    <div className="border border-gray-200 rounded-lg p-4 hover:shadow-lg">
      ...
    </div>
  );
}

CSS Approach Comparison

Approach	Scoped?	Bundle Size	Learning Curve	Best For
Regular CSS	❌ Global	Small	Low	Simple projects
CSS Modules	✅ Scoped	Small	Low	Medium projects
Styled Components	✅ Scoped	Medium	Medium	Component libraries
Tailwind CSS	✅ Utility	Small (purged)	Medium	Rapid development

How These Languages Work Together in React

Here’s how JavaScript, JSX, TypeScript, HTML concepts, and CSS combine in a real React component:

tsx

// TypeScript + JSX + CSS Modules
// File: ProductCard.tsx

import { useState } from 'react';          // JavaScript (React hook)
import styles from './ProductCard.module.css'; // CSS Module

// TypeScript interface
interface Product {
  id: number;
  name: string;
  price: number;
  image: string;
  inStock: boolean;
}

interface ProductCardProps {
  product: Product;
  onAddToCart: (id: number) => void;
}

// JSX component (HTML-like structure + JavaScript logic)
export default function ProductCard({ product, onAddToCart }: ProductCardProps) {
  const [quantity, setQuantity] = useState(1);   // JavaScript state

  // JavaScript event handler
  const handleAdd = () => {
    if (product.inStock) {
      onAddToCart(product.id);
    }
  };

  // JSX return (HTML structure + JS expressions + CSS classes)
  return (
    <div className={styles.card}>
      <img src={product.image} alt={product.name} className={styles.image} />
      <h3 className={styles.name}>{product.name}</h3>
      <p className={styles.price}>${product.price.toFixed(2)}</p>
      
      {product.inStock ? (
        <div className={styles.actions}>
          <select 
            value={quantity} 
            onChange={(e) => setQuantity(Number(e.target.value))}
          >
            {[1, 2, 3, 4, 5].map(n => (
              <option key={n} value={n}>{n}</option>
            ))}
          </select>
          <button onClick={handleAdd} className={styles.addButton}>
            Add to Cart
          </button>
        </div>
      ) : (
        <p className={styles.outOfStock}>Out of Stock</p>
      )}
    </div>
  );
}

Language Breakdown of This Component

Line	Language/Technology	Purpose
import { useState }	JavaScript (ES6 modules)	Import React hook
import styles	JavaScript + CSS Modules	Import scoped styles
interface Product	TypeScript	Define data shape
function ProductCard	JavaScript	Component declaration
({ product, onAddToCart })	JavaScript (destructuring)	Extract props
useState(1)	JavaScript (React API)	State management
<div className={styles.card}>	JSX + CSS	HTML structure + styling
{product.name}	JavaScript expression in JSX	Dynamic content
{product.inStock ? (...) : (...)}	JavaScript (ternary)	Conditional rendering
.map(n => ...)	JavaScript (array method)	List rendering
onChange={(e) => ...}	JavaScript (event handler)	User interaction

Every technology plays a specific role: JavaScript provides logic, JSX provides structure, TypeScript provides safety, and CSS provides styling.

JavaScript vs. TypeScript for React: Which Should You Use?

This is one of the most common questions for React developers. Here’s the definitive comparison:

Factor	JavaScript	TypeScript
Learning curve	Lower (no type system to learn)	Higher (must learn types, interfaces, generics)
Development speed (initial)	Faster to start	Slower setup, but catches errors early
Development speed (long-term)	Slows down with debugging	Faster with autocomplete + error prevention
Error detection	Runtime only (errors in production)	Compile-time (errors during development)
IDE support	Good	Excellent (superior autocomplete)
Team projects	Harder to maintain	Self-documenting, easier onboarding
Community adoption	Declining for new React projects	78% of React devs use it (2024)
Best for	Prototypes, small projects, learning	Production apps, teams, large codebases

Recommendation

• Learning React? Start with JavaScript. Understand React fundamentals first, add TypeScript later.
• Building production apps? Use TypeScript from day one. The upfront investment pays off quickly.
• Working on a team? TypeScript is almost mandatory. It prevents entire categories of bugs.

The Complete React Technology Stack

React doesn’t work alone. Here’s the full ecosystem of languages and tools involved:

Core (Required)

Technology	Role	Type
JavaScript	Core programming language	Language
JSX	Component syntax	Syntax extension
HTML (via JSX)	DOM structure	Markup
CSS	Visual styling	Stylesheet

Recommended Additions

Technology	Role	Type
TypeScript	Type safety	Language superset
Node.js	Development tooling, build process	Runtime
npm/yarn/pnpm	Package management	Tool
Vite or Webpack	Bundling and dev server	Build tool
Babel	JSX compilation, JS transpilation	Compiler

Common Ecosystem Libraries

Library	Purpose
React Router	Client-side navigation
Redux / Zustand	Global state management
TanStack Query	Server state / data fetching
Tailwind CSS	Utility-first styling
Next.js	Full-stack React framework
React Testing Library	Component testing

For a complete overview of what you can build with this technology stack, explore our guide on what ReactJS is used for in modern web development.

What You Need to Learn (In Order)

If you’re starting from scratch, here’s the exact learning path:

Phase 1: JavaScript Fundamentals (2–4 weeks)

Must-know topics:

• Variables (const, let), data types, operators
• Functions (declarations, expressions, arrow functions)
• Arrays and objects (creation, methods, iteration)
• Control flow (if/else, switch, loops)
• ES6+ features (destructuring, spread, template literals, modules)
• Asynchronous JavaScript (promises, async/await, fetch)
• DOM basics (understanding what React abstracts)

Resources:

• JavaScript.info — Comprehensive, free
• MDN JavaScript Guide — Official reference
• freeCodeCamp JavaScript — Interactive exercises

Phase 2: React Core (2–3 weeks)

Must-know topics:

• Components (functional components, JSX syntax)
• Props (passing data, prop types)
• State (useState, lifting state up)
• Effects (useEffect, cleanup functions)
• Event handling (forms, clicks, keyboard)
• Conditional rendering (ternary, &&, early returns)
• List rendering (.map(), key prop)

Resources:

• React.dev — Official interactive tutorial
• Scrimba React Course — Free video course

Phase 3: React Ecosystem (2–4 weeks)

Must-know topics:

• React Router (navigation, URL parameters)
• Data fetching patterns (TanStack Query or SWR)
• State management (Context API, then Redux or Zustand)
• CSS approach (pick one: Modules, Tailwind, or Styled Components)
• TypeScript with React (interfaces, generics, typed hooks)

Phase 4: Production Skills (Ongoing)

• Testing (React Testing Library, Jest)
• Performance optimization (memoization, code splitting)
• Next.js or Remix (server-side rendering)
• Deployment (Vercel, Netlify, AWS)

Understanding where React sits in the web development stack helps you plan what backend technologies to pair it with.

FAQs

Is JavaScript the only language used in ReactJS development?

JavaScript is the core language. However, most production React projects use TypeScript (a typed superset of JavaScript) for better code safety and developer experience. You also use HTML concepts (through JSX) and CSS for styling. But the programming logic is always JavaScript or TypeScript.

Can I use JSX without JavaScript?

No. JSX is a syntax extension of JavaScript. It compiles into React.createElement() calls, which are JavaScript functions. Without JavaScript knowledge, JSX code won’t make sense, and you won’t be able to add logic, handle events, or manage state.

Can I write React without JSX?

Technically yes, but practically no. You can use React.createElement() directly, but it’s extremely verbose. A simple component that takes 5 lines in JSX would take 20+ lines without it. Every React developer uses JSX.

Are there alternatives to JavaScript for ReactJS?

TypeScript is the main alternative it’s a superset of JavaScript that adds static types. Some experimental tools like ReScript (formerly BuckleScript) and Elm can compile to React-compatible JavaScript, but they have tiny ecosystems compared to JavaScript/TypeScript.

Do I need to know HTML before learning React?

Yes. JSX mirrors HTML structure. If you don’t understand HTML elements (div, form, input, table, section), their attributes, and semantic meaning, you’ll struggle to write correct JSX. Spend a few days learning HTML basics before starting React.

Do I need to know CSS before learning React?

Basic CSS is required. You need to understand selectors, properties (colors, spacing, layout), flexbox, and grid. You don’t need to be a CSS expert, but you should be comfortable styling elements. React provides many CSS approaches (modules, Tailwind, styled-components) that build on these fundamentals.

Is TypeScript replacing JavaScript for React?

TypeScript is becoming the default, but it’s not replacing JavaScript TypeScript is JavaScript with types added. Every valid JavaScript code is valid TypeScript. The trend is clear: 78% of React developers use TypeScript in production (2024), and all major React frameworks (Next.js, Remix) default to TypeScript templates.

What’s the difference between JavaScript and JSX?

JavaScript is a programming language. JSX is a syntax extension that lets you write HTML-like code inside JavaScript. JSX gets compiled into standard JavaScript (React.createElement() calls) before execution. Think of JSX as a more readable way to write JavaScript UI code.

Can I use Python, Java, or C++ with React?

Not directly. React runs in the browser, which only executes JavaScript. However, Python, Java, or C++ can power the backend API that a React frontend communicates with. For example, a React frontend + Django (Python) backend is a common architecture.

For a deeper understanding of the technical distinctions between React and programming languages, our article on whether React is a language or a library covers the architectural differences in detail.

Conclusion: JavaScript is React’s Language Master It First

ReactJS is built entirely on JavaScript, extended by JSX for readable component syntax, optionally enhanced with TypeScript for type safety, and styled with CSS. Every React concept components, state, props, hooks, effects is JavaScript underneath.

The Core Takeaways

1. JavaScript is non-negotiable — You must learn JavaScript before React
2. JSX is syntactic sugar — It compiles to JavaScript, not a separate language
3. TypeScript is the modern standard — 78% adoption and growing
4. HTML/CSS knowledge is required — JSX mirrors HTML; styling needs CSS
5. The ecosystem is JavaScript-native — All React tools, libraries, and frameworks use JavaScript

The Practical Implication

The single most impactful thing you can do to become a better React developer is master JavaScript deeply. Developers who understand closures, prototypes, event loops, and async patterns write significantly better React code than those who memorized React patterns without understanding the language beneath them.

Continue Learning About React

This article is part of our React Fundamentals series:

• What is ReactJS Used For? Complete Guide — Every major React application and use case
• Is ReactJS a Programming Language? — Why React is a library, not a language
• Is React a Language or a Library? — Architectural and technical distinctions
• Is React Front-End or Backend? — Where React fits in the full web development stack
• Why ReactJS Makes a Perfect Choice for Your Next Project — Decision framework for choosing React

External Resources

• React Official Documentation
• MDN: JavaScript Reference
• TypeScript Official Documentation
• Babel: JSX Compiler
• JavaScript.info: Modern JavaScript Tutorial
• freeCodeCamp JavaScript Curriculum

About This Content

This article is maintained by Priyanshu Pathak, Senior Developer at Sourcebae, with hands-on experience building production React applications using JavaScript and TypeScript. Content accuracy verified against:

• Official React.dev Documentation
• MDN Web Docs JavaScript Reference
• TypeScript Official Handbook
• State of JavaScript Survey 2024
• Stack Overflow Developer Survey 2024

Last Updated: June 2026
Update Schedule: Quarterly for new language features and ecosystem changes