diff --git a/src/utils/__tests__/group.spec-d.ts b/src/utils/__tests__/group.spec-d.ts
index d58e9c5b..3c1bd545 100644
--- a/src/utils/__tests__/group.spec-d.ts
+++ b/src/utils/__tests__/group.spec-d.ts
@@ -7,11 +7,11 @@ import type Vehicle from '#fixtures/types/vehicle'
 import type testSubject from '../group'
 
 describe('unit-d:utils/group', () => {
-  it('should return { [H in K]?: T[number][] }', () => {
+  it('should return { [H in K]: T[number][] }', () => {
     // Arrange
     type T = Vehicle[]
     type K = Vehicle['year']
-    type Expect = { [H in K]?: T[number][] }
+    type Expect = { [H in K]: T[number][] }
 
     // Expect
     expectTypeOf<typeof testSubject<T, K>>().returns.toEqualTypeOf<Expect>()
diff --git a/src/utils/__tests__/group.spec.ts b/src/utils/__tests__/group.spec.ts
index 70bdaf6e..318432f9 100644
--- a/src/utils/__tests__/group.spec.ts
+++ b/src/utils/__tests__/group.spec.ts
@@ -14,8 +14,8 @@ describe('unit:utils/group', () => {
     ]
 
     // Act + Expect
-    cases.forEach(([array, identity, expected]) => {
-      expect(testSubject(array, identity)).to.eql(expected)
+    cases.forEach(([arr, key, expected]) => {
+      expect(testSubject(arr, key)).to.eql(expected)
     })
   })
 })
diff --git a/src/utils/group.ts b/src/utils/group.ts
index 23403763..4d34e40c 100644
--- a/src/utils/group.ts
+++ b/src/utils/group.ts
@@ -4,14 +4,16 @@
  */
 
 import type { Mapper, PropertyKey } from '#src/types'
+import cast from './cast'
+import isUndefined from './is-undefined'
 
 /**
- * Groups each item in an array.
+ * Group each item in an array.
  *
  * The return value is a plain object with a key for each group, where each key
  * value is an array containing group items.
  *
- * A `mapper` function is used to convert array items to group keys.
+ * A `key` function is used to map array items to group keys.
  *
  * @see {@linkcode Mapper}
  *
@@ -21,32 +23,32 @@ import type { Mapper, PropertyKey } from '#src/types'
  * @template K - Identity key type
  *
  * @param {T} arr - Array to group
- * @param {Mapper<T, K>} mapper - Array item interpolator
- * @return {Partial<Record<K, T[number][]>>} Groups object
+ * @param {Mapper<T, K>} key - Group key mapper
+ * @return {Record<K, T[number][]>} Groups object
  */
 const group = <
   T extends readonly unknown[],
   K extends PropertyKey = PropertyKey
 >(
   arr: T,
-  mapper: Mapper<T, K>
-): { [H in K]?: T[number][] } => {
-  return arr.reduce<{ [H in K]?: T[number][] }>((acc, item, i) => {
+  key: Mapper<T, K>
+): { [H in K]: T[number][] } => {
+  return arr.reduce<{ [H in K]: T[number][] }>((acc, item, i) => {
     /**
      * Group key.
      *
-     * @const {K} key
+     * @const {K} k
      */
-    const key: K = mapper(item, i, arr)
+    const k: K = key(item, i, arr)
 
     // initialize group
-    !acc[key] && (acc[key] = [])
+    isUndefined(acc[k]) && (acc[k] = [])
 
     // add group item
-    acc[key]!.push(item)
+    acc[k].push(item)
 
     return acc
-  }, {})
+  }, cast({}))
 }
 
 export default group