feat: add groupByKey utility

src/shared/lib/groupByKey.test.ts

@@ -0,0 +1,54 @@
+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' }],
+      });
+    });
+  });
+});