隠れ身の術

#emoji {
  font-size: 60px;
  transition: opacity 3s;
}

const emoji = document.querySelector('#emoji');
const btn = document.querySelector('#btn');
let isVisible = true;

btn.addEventListener('click', function() {
  if (isVisible) {
    emoji.style.opacity = '0';
    isVisible = false;
  } else {
    emoji.style.opacity = '1';
    isVisible = true;
  }
});