why is filter null

module.exports = function(directory, filter, foo) {
        var fs = require('fs');

        fs.readdir(directory, function read(err, data) {
                if(err) {
                        return foo(err);
                }

                var directorylist = data.toString().split(',');

                var directoryfilter = [];

                for(var i=0; i<directorylist.length; i++) {
                        var filter = directorylist[i];

                        var filtersplit = filter.split('.');


                        if(filtersplit[1]  == filter) {
                                directoryfilter.push(filter);
                        }
                }

                foo(null, directoryfilter);
        });
};