.classList

const modal = document.querySelector('div');
modal.classList.add('modal');

const inner = document.createElement('div');
inner.classList.add('inner');

modal.appendChild(inner);

const btn = document.querySelector('.btn');
btn.addEventListener('click', ()=>{
  document.body.appendChild(modal);
});

inner.addEventListener('click', ()=>{
  document.body.removeChild(modal);
});