condense ISBN generator

frontend/files/index.html

@@ -158,14 +158,7 @@
         ISBN = EAN.slice(3, 12);
         var checkdigit =
           (11 -
-            (_.reduce(
-              ISBN.split(""),
-              function (sum, num, key) {
-                return sum + num * (10 - key);
-              },
-              0
-            ) %
-              11)) %
+            (_.reduce(ISBN.split(""), (s, n, k) => s + n * (10 - k), 0) % 11)) %
           11;
         return ISBN + (checkdigit == 10 ? "X" : checkdigit);
       }