React-functional-component.js

 function CounterFunction() {
  const [count, setCount] = React.useState(1);
  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => 
        setCount(count + 1)}
      >Increase</button>
    </div>
  );
}

ReactDOM.render(
  <div> 
    <CounterFunction />
  </div>
, document.querySelector('#app'));