Components, props, and state. Hooks (useState, useEffect). Single Page Apps — fast, fluid, no full refresh.
// React component with hooks
function CoffeeCard({ name, price }) {
const [count, setCount] = useState(0);
useEffect(() => {
document.title = `${count} in cart`;
}, [count]);
return <div>{name} — ${price}</div>;
}Props flow down, state stays local. Lift state up when siblings need to share.
Single Page App = one HTML load, then JS handles routing and updates.