Getting object when is ready

I have a module, that fetches some object. Then other modules can use this object. Here is a little example of this task:

var Structure = (function(){

    var tree = null,

        df = $.ajax({
            'url'       : '/some/path',
            'type'      : 'POST',
            'data'      : { some : 'data' },
            'dataType'  : 'json'
        }),

        ready = function(cb){
            if (tree) {
                cb(tree);
                return
            }

            $.when(df).then(function(data){
                tree = TreeHandler.create(data);
                cb(tree);
            });
        };

    return {
        df      : df,
        ready   : ready
    };

})();

// Usage:
Structure.ready(function(data){
     console.log (data);
     // some actions with data
});

Am I right on doing this? How can this be made better?

I'm repeating @RoToRa for the sake putting it in answer form for beta-stats:

Your syntax is fine. Without knowing more of the problem domain, it does seem a little unnecessary. Why not use callbacks on your $.ajax() call?

StackExchange Code Review Q#7198, answer score: 3