使用闭包在javascript中枚举对象属性的静态变量

所以我一直在研究javascript，现在我正在关注闭包 . 我以为我理解了所有的概念，但我显然错过了一些东西 . 出于学习目的，我创建了这个闭包，它基本上就像一个带有Object和整数成员的类 . 代码如下：

var AllFilters = (function () {

        var _length = 0;
        var _filters = {};

        //Return new length.
        //Overwrites if exists.
        function _addFilter(filter) {
            _filters[filter.name] = filter;
            _length++; //test if it was actually incremented...
        }

        //Remove property from object.
        //Returns the removed propety value, or undefinned.
        function _removeFilter(filter) {

            var removed;

            if (filter.name !== undefined) {
                //Removed based on filter object;
                removed = _filters[filter.name];
                delete _filters[filter.name];
            } else {
                //Removed based on filter name;
                removed = _filters[filter];
                delete _filters[filter];
            }

            _length--; //teste if it was actually deleted...
            return removed;
        }

        //Param = filter name;
        //Returns undefined or filter;
        function _getFilter(filter) {
                return _filters[filter];
        }

        //Return all filters in an array.
        function _getAsArray() {

            var arr = [];

            for (var filter in _filters) {

                if (_filters.hasOwnProperty(filter)) {
                    arr.push(_filters[filter]);
                }
            }

            return arr;
        }

        return function () {

            return {

                addFilter: _addFilter,

                removeFilter: _removeFilter,

                getFilter: _getFilter,

                getAsArray: _getAsArray,

                length: _length

            };
        };

    }());

使用：

x = AllFilters();
x.addFilter({ name: "a", foo: "bar" });
x.addFilter({ name: "b", foo: "baz" });
x.addFilter({ name: "c", foo: "qux" });

一切都很完美，除了长度：它总是返回零，我想让它返回当前值，就像静态var . _filters var正确返回，它并不总是像长度一样重置，所以我真的不知道这里发生了什么，我认为_length会像_filters一样保持它的状态 . 有什么想法吗？