test: add groupBy polyfill tests

test/groupBy.test.ts

@@ -0,0 +1,57 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { polyfillGroupBy } from '../src/polyfill'
+
+describe('polyfillGroupBy', () => {
+  beforeEach(() => {
+    if ('groupBy' in Object) {
+      delete (Object as { groupBy?: unknown }).groupBy
+    }
+    polyfillGroupBy()
+  })
+
+  it('should define Object.groupBy if not already defined', () => {
+    expect(Object.groupBy).toBeDefined()
+  })
+
+  it('should group array of objects by a given key', () => {
+    const array = [
+      { id: 1, category: 'fruit', name: 'apple' },
+      { id: 2, category: 'vegetable', name: 'carrot' },
+      { id: 3, category: 'fruit', name: 'banana' },
+    ]
+    const result = Object.groupBy(array, item => item.category)
+    expect(result).toEqual({
+      fruit: [
+        { id: 1, category: 'fruit', name: 'apple' },
+        { id: 3, category: 'fruit', name: 'banana' },
+      ],
+      vegetable: [{ id: 2, category: 'vegetable', name: 'carrot' }],
+    })
+  })
+
+  it('should return an empty object for an empty array', () => {
+    const result = Object.groupBy([], () => 'key')
+    expect(result).toEqual({})
+  })
+
+  it('should group by a dynamic key', () => {
+    const array = [
+      { id: 1, category: 'fruit', name: 'apple' },
+      { id: 2, category: 'fruit', name: 'banana' },
+      { id: 3, category: 'vegetable', name: 'carrot' },
+    ]
+    const result = Object.groupBy(array, item => item.name[0])
+    expect(result).toEqual({
+      a: [{ id: 1, category: 'fruit', name: 'apple' }],
+      b: [{ id: 2, category: 'fruit', name: 'banana' }],
+      c: [{ id: 3, category: 'vegetable', name: 'carrot' }],
+    })
+  })
+
+  it('should not overwrite existing Object.groupBy', () => {
+    Object.groupBy = vi.fn()
+    polyfillGroupBy()
+    expect(Object.groupBy).toBeInstanceOf(Function)
+    expect(Object.groupBy).toHaveBeenCalledTimes(0)
+  })
+})