load stepper for async functions

loader.js

    var a = function(callback) { console.log("A"); callback("Hello from A"); };
    var b = function(response, callback) { if (!response) { throw "Err: no response"; } console.log("B", response); callback(); };
    var c = function(callback) { console.log("C"); callback(); };
    var d = function() { console.log("D"); };
    var e = function(callback) { console.log("E"); callback(); };
    var f = function() { console.log("F"); };

    var load = (function() {
      var funcs = [];
      var index = 0;

      var _Stepper = {
        run: function() {
          this.next();
        },

        next: function() {
          var func = funcs[index];
          if (func && typeof func.func === "function") {
            console.log("Running <" + func.name + ">");
            
            index++;            

            func.func.apply(this, arguments);

            if (!func.async) {
              _Stepper.next.apply(this, arguments);
            }
          } else {
            console.log("END");
          }
        },

        add: function(name, func, async) {
          funcs.push({ name: name, func: func, async: async });
        }
      };

      return _Stepper;
    }());

    load.add("A", function() { a(load.next); }, true);
    load.add("B", function(response) { b(response, load.next); }, true);
    load.add("C", function() { c(load.next); }, true);
    load.add("D", function() { d(); });
    load.add("E", function() { e(load.next); }, true);
    load.add("F", function() { f(); }, true);
    
    load.run();