code: function digitalInvoke(counter, cb) {
      var newcounter = counter - 1;
      if (newcounter <= 0) {
          if (!!cb && typeof cb == "function") {
              cb.apply(null, Array.prototype.slice.call(arguments, 2));
          }
      }
      return newcounter;
  }
docs: 回调计数
  
  @param {Number} counter
  @param {Function} cb
  @returns {Number}