sync.js

function sync(result) {
  const { maintained, removed, added, list } = result;
  const prevItems = this.items;
  const newItems = [];
  
  maintained.forEach(([beforeIndex, afterIndex]) => {
    newItems[afterIndex] = prevItems[beforeIndex];
  });
  added.forEach(i => {
    newItems[i] = new Item(list[i]);
  });
  this.items = newItems;
  this.recaculateItems();
}