What is React and why would you use it?

React's core idea is declarative rendering. Instead of manually finding DOM nodes and changing them, you return a description of the UI from a component. When state or props change, React works out what changed and updates the DOM.

function Welcome({ name }) {
  return <h1>Hello, {name}</h1>;
}

function App() {
  return (
    <main>
      <Welcome name="Asha" />
      <Welcome name="Ravi" />
    </main>
  );
}

Why teams use it

• Components make UI reusable and easier to reason about
• Declarative updates remove a large class of manual DOM bugs
• Unidirectional data flow makes state changes predictable
• A large ecosystem covers routing, data fetching, forms and testing
• React Native reuses the same model for mobile

React is a library, not a full framework. It does not prescribe routing, data fetching or project structure. In an interview, say that clearly and explain which surrounding tools you would choose and why.