gistfile1.js

function Something() {
    _.extend(this, {
        attr: 1,
        _extras = {},
        __setattr__: function (key, value) {
            this._extras[key] = value;
        },
        __getattr__: function (key) {
            return this._extras[key] || 'That attribute doesnt exist yet!';
        }
    })
}

var s = new Something();
console.log(s.attr); // 1
console.log(s.x); // 'That attribute doesnt exist yet!'
s.x = 2;
console.log(s.x); // 2