You can use:
var myVar = document.getElementById("iframeid").contentWindow.myVar;
Make sure the domains match and that you're using a web server (if testing using the file system you'll get a protocol warning).
Hello, my name is Lee and I work as a full-stack web developer specialising in Microsoft ASP.NET technologies. I love using Umbraco and also MANAGED, my own application management software.
Contact me at lee.gunn@secretorange.co.uk
Get in touch to talk about your project or just ask me a question.