Александр

localStorage.setItem('key', 'Value');
localStorage.getItem('key'); // "Value"

form.addEventListener('submit', function(evt) {
  evt.preventDefault();

  // ничего не делаем
  if (document.querySelector('#hidden').value) return

  let formData = {
    name: document.querySelector('input[name="name"]').value,
    phone: document.querySelector('input[name="phone"]').value,
    email: document.querySelector('input[name="email"]').value,
    textarea: document.querySelector('textarea[name="textarea"]').value,
  };

  var request = new XMLHttpRequest();

SELECT items.id, items.name, IFNULL(count(details.id), 0)
FROM items
LEFT JOIN details ON items.id = details.item_id
GROUP BY items.id