Generated javascript ignores the "optional" keyword

The "optional" keyword is ignored in the generated javascript.

Given a thrift structure like :
>>
struct Contract {
1: SummaryContract summary,
3: string guarantees,
4: string options,
5: optional VehicleContract vehicle,
6: optional ResidenceContract residence,
7: optional HealthContract health,
}
>>

The thrift with JSONProtocol sent by the java server looks like :
>>
.."3":

{"str":"VEHICLE CONTRACT 1"}

}},"3":

{"str":"YAMAHA GUARANTEES"}

,"4":

{"str":"YAMAHA OPTIONS"}

,.."5":{"rec":{"1":

{"i32":2}

,"2":

{"str":"Yamaha"}

,"3":

{"str":"F6"}

,"4":{"str":"ARG-10-MESSI"}}..
>>
the ResidenceContract and HealthContract are structures are not sent by the java server.

But when this thrift stream is read by the javascript, the result is a json structure like :

>>
..,"description":"VEHICLE CONTRACT 1"},"guarantees":"YAMAHA GUARANTEES","options":"YAMAHA OPTIONS","vehicle":

{"type":2,"brand":"Yamaha","model":"F6","inmatriculation":"ARG-10-MESSI"}

,"residence":

{"type":0,"building":0,"rooms":0}

,"health":{"type":0}}..
>>

The ResidenceContract and HealthContract JSON objects are present but empty.

This is because the generated javascript is like:

>>
Contract = function(args){
this.summary = new SummaryContract()
this.guarantees = ''
this.options = ''
this.vehicle = new VehicleContract()
this.residence = new ResidenceContract()
this.health = new HealthContract()
>>

we see that the members are created by default in the constructor, even if they are "optional".