code: function(holder, options) {
              var self = this;
              if (!holder) {
                  throw "构造 numbox 时缺少容器元素";
              }
              self.holder = holder;
              options = options || {};
              options.step = parseInt(options.step || 1);
              self.options = options;
              self.input = $.qsa(inputClassSelector, self.holder)[0];
              self.plus = $.qsa(plusClassSelector, self.holder)[0];
              self.minus = $.qsa(minusClassSelector, self.holder)[0];
              self.checkValue();
              self.initEvent();
          }
docs: 构造函数