What Is React?

React is an open-source JavaScript library for building user interfaces, first released by Facebook (now Meta) in 2013 and maintained today by Meta and a large open-source community. If you have used Instagram on the web, WhatsApp Web, or countless dashboards and admin panels, you have almost certainly used something built with React.

For anyone learning front-end development in India, React matters for one blunt reason: it is the single most requested front-end skill in job descriptions, and that has been true for years. Learning it well is one of the most direct paths from "I know some HTML and JavaScript" to "I am employable as a developer".

What problem React actually solves

Before React, building interactive pages meant manually finding elements and updating them by hand. You would write code like document.getElementById("count").innerText = newValue scattered across the page, and as apps grew, keeping the screen in sync with your data became a nightmare of tangled updates.

React flips this around. You describe what the screen should look like for any given state of your data, and React figures out how to update the actual page. You stop writing step-by-step DOM instructions and start describing outcomes. This is called a declarative approach, and it is the core idea that makes React click once it lands.

Pro tip: If you remember only one sentence about React, make it this: "UI is a function of state." Give React your data, and it produces the matching interface. Change the data, and React re-renders the affected parts. Interviewers love candidates who can say this and explain it.

Components: the building blocks

Everything in React is a component — a reusable, self-contained piece of UI. A button, a search bar, a product card, or an entire page can each be a component. You build big interfaces by composing small components together, the same way you build with Lego bricks.

A modern React component is just a JavaScript function that returns markup. Here is a complete, working example:

function Welcome({ name }) {
  return <h2>Hello, {name}. Welcome to CodeBegun!</h2>;
}

function App() {
  return (
    <div>
      <Welcome name="Aravind" />
      <Welcome name="Samyuktha" />
    </div>
  );
}

export default App;

Notice how Welcome is defined once and reused twice with different data. That reuse is the heart of React. The { name } part is a prop — the way a parent passes data down to a child. The HTML-looking syntax is JSX, which we cover in depth in the JSX syntax guide.

JSX: HTML inside JavaScript

That <h2>Hello, {name}</h2> line is not a string and it is not real HTML — it is JSX, a syntax extension that lets you write markup directly in your JavaScript. A build tool converts it into regular function calls before it reaches the browser.

JSX feels strange for about a day and then becomes natural. The curly braces { } are where you drop in any JavaScript expression: a variable, a calculation, a function call. This tight blend of logic and markup is exactly why React code reads cleanly once you are used to it.

The virtual DOM: why React is fast

Updating the real browser DOM is slow, especially when done repeatedly. React solves this with a virtual DOM — an in-memory JavaScript representation of your UI.

When your data changes, React does three things. First, it builds a fresh virtual DOM tree describing the new UI. Second, it compares (or "diffs") that new tree against the previous one. Third, it calculates the smallest set of real DOM changes needed and applies only those. This process is called reconciliation.

The practical result: you write code as if you are redrawing the whole screen on every change, but React quietly updates only the handful of elements that actually differ. You get simple mental models and good performance at the same time.

Interview note: A classic fresher question is "Why does React use a virtual DOM?" The strong answer names both benefits: it makes updates efficient by minimizing real DOM operations, and it lets you write declarative code without manually tracking what changed. Do not stop at "it is faster" — explain the diffing.

A slightly more real example: state

Static components are only half the story. Real interfaces respond to the user, and for that React uses state — data that can change over time and triggers a re-render when it does. Here is a counter using the useState hook:

import { useState } from "react";

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

  return (
    <button onClick={() => setCount(count + 1)}>
      Clicked {count} times
    </button>
  );
}

export default Counter;

Every click calls setCount, which updates the state. React notices the change, re-runs the Counter function, and updates the button text — all automatically. You never touched the DOM. Hooks like useState are the modern way to add behavior to components, and we introduce all the core ones in the React hooks introduction.

Library, not framework

You will often hear React called a framework, but strictly speaking it is a library focused on one job: rendering UI. It deliberately leaves other concerns to separate tools:

This modular design is a strength: you pick the pieces you need. It also means "learning React" really means learning React plus a handful of companion tools. When you are ready to set up a real project, the Vite setup guide walks you through the standard starting point used in the industry today.

Where React fits in a full-stack career

React handles the front end — what users see and click. On its own it does not talk to a database or handle business logic; that is the back end's job. A complete web application pairs a React front end with a server, most often Spring Boot (Java) or Node.js.

In Hyderabad's hiring market, two React-heavy profiles dominate. The first is Java full stack, where React sits on top of a Spring Boot backend — the exact combination our Java Full Stack course prepares you for. The second is the JavaScript-only MERN stack. Both start with solid React, which is why we treat it as a core skill rather than an optional extra.

Common mistake: Beginners often rush into React tutorials before their JavaScript is ready, then blame React for confusion that is really a JavaScript gap. If arrow functions, array .map(), destructuring, and the spread operator feel shaky, spend a week strengthening those first. React will feel dramatically easier.

How React updates the screen, step by step

Putting the pieces together, here is the full loop that happens on every interaction:

1. The user does something — clicks, types, or the app receives data.
2. An event handler updates state with a setter like setCount.
3. React re-runs the affected component function to produce new JSX.
4. React builds a new virtual DOM tree and diffs it against the old one.
5. React applies only the minimal real DOM changes needed.

Understanding this cycle is what separates people who can "copy React code" from people who can debug and reason about it. Almost every tricky React bug comes down to a misunderstanding of when and why a component re-renders.

Your next steps with React

React is not something you learn by reading — it is something you learn by building. A realistic first sequence looks like this: set up a project with Vite, get comfortable with JSX and components, learn props and state, then learn the core hooks. After that, add routing and forms to build something real.

A good starting project is a to-do list or a simple product catalog. Small, boring apps teach you the fundamentals far better than trying to clone a big product on day one.

If you prefer a guided path with milestones and interview checkpoints, our React learning hub sequences everything, and the beginner React tutorial on the blog walks through your first app end to end. When you are ready to think about the job itself, the how to become a React developer guide maps the full path from beginner to hired. Your immediate next read, though, is the Vite setup guide so you can run real code instead of just reading about it.