Using states in functional components.

usestate.js

function App() {
  const [count, setCount] = React.useState(0)

  function increment() {
    setCount(prevCount => prevCount + 1)
  }

  function decrement() {
    setCount(prevCount => prevCount - 1)
  }

  return (
    <div>
      <h1>{count}</h1>
      <button onClick={increment}>Increment!</button>
      <button onClick={decrement}>Decrement!</button>
    </div>
  )
}

ReactDOM.render(<App />, document.getElementById("root"))