diff --git a/GuessNN-webapp/src/Utils.ts b/GuessNN-webapp/src/Utils.ts
index 0b6e5bd..776df14 100644
--- a/GuessNN-webapp/src/Utils.ts
+++ b/GuessNN-webapp/src/Utils.ts
@@ -6,3 +6,14 @@ export function fetchJSON(url: string): Promise<unknown> {
 			return response.json();
 		});
 }
+
+export function groupBy<V, K>(list: Iterable<V>, mapper: (_el: V) => K): Map<K, V[]> {
+	const result = new Map();
+	for (const el of list) {
+		const key = mapper(el);
+		if (!result.has(key))
+			result.set(key, []);
+		result.get(key).push(el);
+	}
+	return result;
+}