spec.js

define([
    'msgme/user-preferences'
], function(userPreferences) {
    describe('user preferences', function () {
        afterEach(function () {
            userPreferences.clear();
        });

        describe('module', function () {
            it('is a function that returns a promise', function () {
                expect(userPreferences).to.be.a(Function);
                expect(userPreferences()).to.have.property('then');
            });

            it('resolves to a RecordModel-like instance', function (done) {
                userPreferences().then(function(prefs) {
                    expect(prefs).to.have.property('save');
                    expect(prefs).to.have.property('del');
                    done();
                });
            });
        });
    });
});

user-preferences.js

define([
    'waterfall/record',
    'msgme/ko',
    'msgme/viewmodel/mapping',
    'json!widgets/shared-strings.json'
], function (record, ko, mapping, sharedStrings) {
    var recordModel;
    var version = sharedStrings.version;
    var KEY = 'waterfall-mobile-user-prefs-' + version.major + '.' +
        version.minor + '.' + version.patch;

    function LocalRecordModel() {
        mapping.RecordModel.apply(this, arguments);
    }

    LocalRecordModel.prototype.save = function () {
        localStorage.setItem(KEY, ko.mapping.toJS(this));
        return $.Deferred().resolve(this);
    };

    LocalRecordModel.prototype.del = function () {
        accessor.clear();
        return $.Deferred().resolve();
    };

    LocalRecordModel.create = function() {
        return new LocalRecordModel(
            new record.Record(localStorage.getItem(KEY) || {}, {url: 'local'}));
    };

    function accessor() {
        if (!recordModel) {
            recordModel = LocalRecordModel.create();
        }

        return $.Deferred().resolve(recordModel);
    }

    accessor.clear = function () {
        recordModel = null;
        localStorage.removeItem(KEY);
    };

    return accessor;
});