title: How to customize the message "Changes you made may not be saved." for window.onbeforeunload?
body: <p>I am testing in Google Chrome.</p>
  
  <p>I did some search and found that someone is using:</p>
  
  <pre><code>window.onbeforeunload = function() {
      if (hook) {
        return "Did you save your stuff?"
      }
    }
  </code></pre>
  
  <p>But when I use it, I still got the "Changes you made may not be saved." message. How can I change it to something I want?</p>