<div id="box">
  This is div
</div>

<button id="add-btn">Add class</button>
<button id="remove-btn">Remove class</button>

.shadow{
  box-shadow: 2px 1px 2px 2px #ccc;
  padding:1rem;
  margin:1rem;
}

const div = document.getElementById('box');
const addBtn = document.getElementById('add-btn');
const removeBtn = document.getElementById('remove-btn');

addBtn.addEventListener('click',()=>{
    div.classList.add('shadow');
})

removeBtn.addEventListener('click',()=>{
    div.classList.remove('shadow');
})