refactor: introduce variable for better debugging

application/source/util/processing.mjs

@@ -86,9 +86,8 @@ class Processing extends Base {
      *
      * So the execution time is timeout1+timeout1+timeout1+timeout2
      *
-     * @param {int} timeout Timeout
-     * @param {function} callback Callback
      * @throw {TypeError} the arguments must be either integer or functions
+     * @param {...(int|function)} args
      */
     constructor(...args) {
         super();
@@ -136,13 +135,17 @@ class Processing extends Base {
      */
     run(data) {
         const self = this;
-        if (this[internalSymbol].queue.isEmpty()) {
+        if (self[internalSymbol].queue.isEmpty()) {
             return Promise.resolve(data);
         }
 
-        return this[internalSymbol].queue
-            .poll()
-            .run(data)
+        const callback = self[internalSymbol].queue.poll();
+
+        if (callback === null || callback === undefined) {
+            return Promise.resolve(data);
+        }
+
+        return callback.run(data)
             .then((result) => {
                 return self.run(result);
             });