test: add unit tests for uniqBy function to verify duplicate handling

Author: dayongkr

src/array/uniqBy.spec.ts

@@ -6,4 +6,22 @@ describe('uniqBy', () => {
     expect(uniqBy([2.1, 1.2, 2.3], Math.floor)).toEqual([2.1, 1.2]);
     expect(uniqBy([1.2, 1.5, 2.1, 3.2, 5.7, 5.3, 7.19], Math.floor)).toEqual([1.2, 2.1, 3.2, 5.7, 7.19]);
   });
+
+  it('should keep the first occurrence when duplicates are found', () => {
+    const items = [
+      { id: 1, value: 'apple' },
+      { id: 2, value: 'banana' },
+      { id: 1, value: 'avocado' },
+      { id: 3, value: 'cherry' },
+      { id: 2, value: 'blueberry' },
+    ];
+
+    const result = uniqBy(items, x => x.id);
+
+    expect(result).toEqual([
+      { id: 1, value: 'apple' },
+      { id: 2, value: 'banana' },
+      { id: 3, value: 'cherry' },
+    ]);
+  });
 });

src/array/uniqBy.ts

@@ -2,6 +2,8 @@
  * Returns a new array containing only the unique elements from the original array,
  * based on the values returned by the mapper function.
  *
+ * When duplicates are found, the first occurrence is kept and the rest are discarded.
+ *
  * @template T - The type of elements in the array.
  * @template U - The type of mapped elements.
  * @param {T[]} arr - The array to process.