added Map.groupBy

src/util.ts

@@ -0,0 +1,11 @@
+export default function groupBy<K, V>(arr: V[], fn: (v: V) => K): Map<K, V[]> {
+  const map = new Map<K, V[]>();
+  arr.forEach(e => {
+    const key = fn(e);
+    if (!map.has(key)) {
+      map.set(key, [] as V[]);
+    }
+    map.get(key)!.push(e);
+  });
+  return map;
+}