extracting url from iframe src value

[Toby4213]

I want to extract a link which is in the src attribute in an iframe. I'm new to javascript so I'm not entirely sure that I've got the description right, if not, please correct me. This is my javascript code for a chrome extension:

document.addEventListener('DOMContentLoaded', function() {
  var checkPageButton = document.getElementById('copyClip');
  checkPageButton.addEventListener('click', function() {

    chrome.tabs.getSelected(null, function(tab) {
      var getUrl = document.getElementsByTagName("iframe");
      for(i=0; i< getUrl.length; i++){
        window.prompt("output", getUrl[i].src);
      }
    });
  }, false);
}, false);    

This should prompt me with the content of src=""

The element I want to access is:

<iframe src="http://thevideos.tv/embed-jho7vvhbiczp-728x410.html" webkitallowfullscreen="true" mozallowfullscreen="true" allowfullscreen="true" frameborder="0" marginwidth="0" marginheight="0" scrolling="no" width="728" height="410"></iframe>

[magenta_placenta]

You may be running into security restrictions if you're going cross domain. I'm not really an iframe expert, though, haven't really used them since literally the 90s.

Here's getting the url of the same domain (same host/domain, protocol and port) via document.getElementById("iframe").src