title: Messaging between page and content script in chrome extension?
body: <p>I am writing a chrome extension which injects codes below to opened page:</p>
  
  <pre><code>s = document.createElement "script"
  s.src = "//example.com/my.js"
  (document.head or document.documentElement).appendChild s
  </code></pre>
  
  <p>So it can load my javascript and run it in the page. But after my.js finishes its job, how can I send a message to my extension?</p>
  
  <p>Thanks!</p>