Get variable from iframe

[Zool]

Is there a way of getting a variable from an iframe to the parent?  There are many ways to pass a variable from the parent to the iframe, but I'm not sure if you can pass a variable from the iframe back to the parent.  When i refresh/update the iframe, i want the updated/new variable "id_no" to be passed to the parent file (so I can put this value in a form field).  I tried the code below, but have been unlucky.

 

document.getElementById("iframe").this.form.id_no.value;

 

Any ideas?