problems(js): add “2631. Group By”

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/js/group-by.js

@@ -0,0 +1,23 @@
+/**
+ * @param {Function} fn
+ * @return {Array}
+ */
+Array.prototype.groupBy = function(fn) {
+    let grouped = new Object();
+
+    for (let x of this) {
+        let key = fn(x);
+
+        if (grouped[key] === undefined) {
+            grouped[key] = new Array();
+        }
+
+        grouped[key].push(x);
+    }
+
+    return grouped;
+};
+
+/**
+ * [1,2,3].groupBy(String) // {"1":[1],"2":[2],"3":[3]}
+ */