import { groupByKey } from './groupByKey';

describe('groupByKey', () => {
  describe('basic grouping', () => {
    it('groups items by a string key', () => {
      const items = [
        { category: 'Frontend', name: 'React' },
        { category: 'Backend', name: 'Node' },
        { category: 'Frontend', name: 'Vue' },
      ];
      expect(groupByKey(items, 'category')).toEqual({
        Frontend: [
          { category: 'Frontend', name: 'React' },
          { category: 'Frontend', name: 'Vue' },
        ],
        Backend: [{ category: 'Backend', name: 'Node' }],
      });
    });

    it('preserves insertion order within each group', () => {
      const items = [
        { category: 'A', order: 1 },
        { category: 'A', order: 2 },
      ];
      expect(groupByKey(items, 'category')['A']).toEqual([
        { category: 'A', order: 1 },
        { category: 'A', order: 2 },
      ]);
    });
  });

  describe('edge cases', () => {
    it('returns empty object for empty array', () => {
      expect(groupByKey<{ category: string }>([], 'category')).toEqual({});
    });

    it('handles all items in same group', () => {
      const items = [
        { type: 'X', id: 1 },
        { type: 'X', id: 2 },
      ];
      const result = groupByKey(items, 'type');
      expect(Object.keys(result)).toHaveLength(1);
      expect(result['X']).toHaveLength(2);
    });

    it('handles single item', () => {
      const items = [{ category: 'Only', name: 'One' }];
      expect(groupByKey(items, 'category')).toEqual({
        Only: [{ category: 'Only', name: 'One' }],
      });
    });
  });
});