diff --git a/test/groupBy.test.ts b/test/groupBy.test.ts
new file mode 100644
index 0000000..64408cd
--- /dev/null
+++ b/test/groupBy.test.ts
@@ -0,0 +1,29 @@
+import { expectType } from 'tsd';
+import { groupBy, __ } from '../es';
+
+// returns optional arrays for the groups
+
+const byGrade = groupBy((student: { score: number; name: string }) => {
+  const score = student.score;
+  return score < 65 ? 'F' : score < 70 ? 'D' : score < 80 ? 'C' : score < 90 ? 'B' : 'A';
+});
+const students = [
+  { name: 'Abby', score: 84 },
+  { name: 'Eddy', score: 58 },
+  { name: 'Jack', score: 69 }
+];
+
+const grouped = byGrade(students);
+expectType<{ score: number; name: string }[] | undefined>(grouped.C);
+(grouped.C ?? []).length;
+// @ts-expect-error
+grouped.C.length;
+
+// accepts a placeholder and later specifying the grouping function
+
+const byGrade2 = groupBy(__, students);
+const grouped2 = byGrade2((student: { score: number; name: string }) => {
+  const score = student.score;
+  return score < 65 ? 'F' : score < 70 ? 'D' : score < 80 ? 'C' : score < 90 ? 'B' : 'A';
+});
+expectType<{ score: number; name: string }[] | undefined>(grouped2.C);
diff --git a/types/groupBy.d.ts b/types/groupBy.d.ts
index d688568..0c9b567 100644
--- a/types/groupBy.d.ts
+++ b/types/groupBy.d.ts
@@ -1,2 +1,5 @@
-export function groupBy<T, K extends string = string>(fn: (a: T) => K): (list: readonly T[]) => Record<K, T[]>;
-export function groupBy<T, K extends string = string>(fn: (a: T) => K, list: readonly T[]): Record<K, T[]>;
+import { Placeholder } from './util/tools';
+
+export function groupBy<T, K extends string = string>(fn: (a: T) => K): (list: readonly T[]) => Partial<Record<K, T[]>>;
+export function groupBy<T>(__: Placeholder, list: readonly T[]): <K extends string = string>(fn: (a: T) => K) => Partial<Record<K, T[]>>;
+export function groupBy<T, K extends string = string>(fn: (a: T) => K, list: readonly T[]): Partial<Record<K, T[]>>;