Андрей

const array = ['part1-item1', 'part1-item2', 'part2-item1', 'part3-item1'];

const result = array.reduce((acc, rec) => {
  const [part, item] = rec.split('-');
  if (!acc[part]) {
    acc[part] = [item];
  } else {
    acc[part].push(item);
  }
  return acc;
}, {});