ON/OFFスイッチ

#status {
  font-size: 2rem;
  color: gray;
  transition: color 0.3s;
}

#status.on {
  color: red;
}

const btn = document.querySelector('#btn');
const status = document.querySelector('#status');
let isOn = false;

btn.addEventListener('click', function() {
  if (isOn) {
    status.textContent = 'OFF';
    status.classList.remove('on');
    isOn = false;
  } else {
    status.textContent = 'ON';
    status.classList.add('on');
    isOn = true;
  }
});