React

…

Tool

Declarative library for building UIs from composable functional components.

Key points

Functional composition: components are pure functions returning JSX from their props.
Hooks (useState, useEffect, useMemo, useCallback) handle state and effects without classes.
Virtual DOM reconciliation: React diffs the tree and commits the minimal set of changes to the real DOM.
Context API + reducers cover most global-state needs without a third-party library.
Server Components cut down client JS; the boundary is marked with 'use client' / 'use server'.
A stable key on lists prevents unnecessary re-renders and component identity bugs.

Code

function Counter() {
  const [count, setCount] = useState(0);
  useEffect(() => {
    document.title = `Count: ${count}`;
  }, [count]);
  return (
    <button onClick={() => setCount((c) => c + 1)}>
      +1 ({count})
    </button>
  );
}

Basic state + effect hook syncing the document title.

References

React Docs
Hooks reference