categorize added/updated/deleted/unchanged items from 2 set

diff.ts

function diff<T>(
    currentItems: T[],
    newItems: T[],
    identifier: (item: T) => any,
    equalComparator: (item1: T, item2: T) => boolean,
  ): { added: T[], updated: T[], deleted: T[], unchanged: T[] } {
    const added: T[] = [];
    const updated: T[] = [];
    const deleted: T[] = [];
    const unchanged: T[] = [];

    const currentItemIds = currentItems.map(i => identifier(i));
    const newItemIds = newItems.map(i => identifier(i));

    newItemIds.forEach((itemId, index) => {
      const currentItemIdIndex = itemId ? currentItemIds.indexOf(itemId) : -1;
      const isNewItem = currentItemIdIndex < 0;
      if (isNewItem) {
        added.push(newItems[index]);
      } else {
        const currentItemVersion = currentItems[currentItemIdIndex];
        const isChanged = !equalComparator(currentItemVersion, newItems[index]);
        if (isChanged) {
          updated.push(newItems[index]);
        } else {
          unchanged.push(currentItemVersion);
        }
      }
    });

    currentItems.forEach((itemId, index) => {
      const newItemIndex = newItemIds.indexOf(itemId);
      const hasItemBeenDeleted = newItemIndex < 0;
      if (hasItemBeenDeleted) {
        deleted.push(currentItems[index]);
      }
    });

    return { added, updated, deleted, unchanged };
  }