document.querySelector('.btn--start').addEventListener('click', filterItems);
document.querySelector('select').addEventListener('change', e => e.target.value === 'all' && filterItems());
function filterItems() {
const val = document.querySelector('select').value;
document.querySelectorAll('.item').forEach(n => {
n.style.display = [ n.dataset.elem, 'all' ].includes(val)
? 'block'
: 'none';
});
}