Static variable to enumarate object properties in javascript using closure

Question

So I've been studying javascript and I'm looking at closures now. I thought I had understood all the concept but I'm obviously missing something. For just studying purposes I created this closure that basically behaves like a class with an Object and an integer member. The code is as follows:

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

            };
        };

    }());

Using:

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

Everything works perfectly, except the length: it always returns zero, I want to make it return the current value, like a static var. The _filters var is returned correctly, it is not always reset like the length, so I really have no idea what is going on here, I thought _length would keep its state just like _filters does. Any thoughts?

Barmar · Accepted Answer · 2014-10-03 04:29:32Z

1

When you update the _length variable it doesn't change the x.length property. That was initialized with the length when you created the object, but you never update it. You need to make that property a function that returns the length from the closure variable:

function _getLength() {
    return _length;
}
...
length: _getLength

Then you can use x.length() to get the current length.

answered Oct 3, 2014 at 4:29

Barmar

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3 Comments

victor Over a year ago

Damn I had tried that, but it didn't work. Now I realized that I was calling x.length instead of x.length() when I put it inside a fcuntion... Still I don't quite get why just using the variable does not work.. If I do filters: _filters and get it as x.filters, the object returned is up to date, so why isnt length?

Barmar Over a year ago

Because a number is not an object. Numbers are copied when assigned, they're not accessed by reference.

victor Over a year ago

I see, so my inner function actually stored a copy of length, not its reference. That makes sense. Thank you!

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