Dependencies in Backbone

example.js

Model = Backbone.Model.extend({
    initialize: function () {
        this.dependentProperty('y', function () {
            return this.get('x') * 100;
        });
    }
});

extension.js

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

you know you really should have blogged this like 2 months ago... it would have saved me countless hours of frustration. :)