Sharing JavaScript model code between server and client, is this approach valid?

I'm beginning learning Node.js and Backbone. I'd like to share some code between the server and the client (related to [1] and [2]). More in deep:

• Share defaults values for a model. Defaults are used to populate forms (client-side), while they are enforced server-side
• Share validation rules in order to be DRY and perform client/server validation
• Share instance methods

So my idea is to use a factory object to encapsulate common code (defaults, validation rules, public instance methods) and adapt the factory object based on the environment (Node.js or AMD module).

This code is quick&dirty and may not work. Is this a good approach or just a waste of time?

(function (factory) {
    if (typeof exports === 'object') { // Node.js
        var config    = require('../config/config.json'),
            Sequelize = require('sequelize'),
                    _ = require('underscore');

        var User = {
            receiveSms: {
                type: Sequelize.BOOLEAN, allowNull: false
            },
            receiveNewsletter: {
                type: Sequelize.BOOLEAN, allowNull: false
            }
        };

        // Add defaultValue for each returned by 
        _.each(factory.defaults, function (value, key) {
            if(_.has(User, key)) {
                User[key].defaultValue = value;
            }
        });

        module.exports = function () {
            (new Sequelize(config.database, config.username, config.password))
                .define('User', User);
        };
    } else if (typeof define === 'function' && define.amd) { // AMD
        define(['backbone', 'uderscore'], function (Backbone, _) {
            return Backbone.Model.extend(factory);
        });
    }
}(function () {
    return { // To be adapted
        defaults: {
            receiveSms: false,
            receiveNewsletter: true
        }
    }
}));