spec/fixtures/pages/window-opener.html (10 lines of code) (raw):
<html>
<body>
<script type="text/javascript" charset="utf-8">
if (window.opener !== null)
window.opener.postMessage(typeof window.opener, '*');
else
require('electron').ipcRenderer.send('opener', window.opener);
</script>
</body>
</html>