import React from 'react';
import ReactDOM from 'react-dom';

function App() {
  let isOn = true
  
  function handleClick() {
    isOn = !isOn
    console.log(isOn)
  }
  
  return <button onClick={handleClick}>{isOn ? "On" : "Off"}</button>
}

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