Style tweaks

server/middleware/utils/createMutex.js

@@ -1,21 +1,18 @@
 function createMutex(doWork) {
   const mutex = {};
 
-  return function(key, payload, callback) {
+  return (key, payload, callback) => {
     if (mutex[key]) {
       mutex[key].push(callback);
     } else {
-      mutex[key] = [
-        function() {
-          delete mutex[key];
-        },
-        callback
-      ];
+      mutex[key] = [callback];
 
-      doWork(payload, function(error, value) {
+      doWork(payload, (error, value) => {
         mutex[key].forEach(callback => {
           callback(error, value);
         });
+
+        delete mutex[key];
       });
     }
   };