How to access element in an iFrame on same page?

I am trying to access a text field value which resides in an iframe:

 function getField(iframeid,iframename,fieldId){
            var mmspobj=parent.document.getElementById(iframeid);
            if (mmspobj.tagName=='IFRAME'){
                 mmsiobj=window.frames[iframename].document.getElementId(fieldId).value;
                 return mmsiobj;
              }
        }

Code (markup):

But that gives the error:

Permission denied to get property Window.document

 

http://www.quirksmode.org/js/frameintro.html

 

If it's not your site shown in the iframe, you'll probably don't have the permission to access it with javascript

 

try this:

alert(window.frames[0].document.getElementById('myId').value);

If you don't manage, message me and i will send you the full example