title: How Can I Return A Value From A postMessage() call asynchronously?
body: <p>I'm trying to get a value from local storage on a site loaded through an Iframe. I'm able to send a message to the site within the iframe through postMessage, but haven't figured out how to return data from the postMessage call. I've looked into promises, but haven't figured out how to get it working. </p>
  
  <p>Anyone have an idea? </p>