Found the newer one

backbone.extensions.js

Backbone.Model.prototype.dependentProperty = function (name, origin, evaluator) {
    var update = function () {
        var value = evaluator.call(this);
        var hash = {};
        hash[name] = value;
        this.set(hash);
    };
    this.bind('change:' + origin, function () {
        update.call(this);
    });

    update.call(this);
};

backbone.extensions.spec.js

describe('BackboneDependentPropertyTester', function () {
    it('should_update_when_target_updates', function () {
        var model = new (Backbone.Model.extend({
            initialize: function () {
                this.dependentProperty('y', 'x', function () {
                    return this.get('x') * 100;
                });
            }
        }));

        model.set({ x: 5 });
        expect(model.get('y')).toEqual(500);
    });
});