getting actual url of frames using javascript

[devloper]

Hi,
I am having a frameset and 2 frames in it.In 1st frame I set the
src property to a URL that is returned by a third party api.
This URL is not the actual url of the page which opens on the frame.
This url internally redirect to the actual URL based on some
parameters in the URL returned by the api.I mean the third party does
not expose us the actual URL.
I need to extract this actual URL in our code. Is there any way to get
it.
Thanks in advance.