How to show something similar to an alert like SO does here with javascript?

Question

You have started writing or editing a post.

Answer 1

you'll need to do ajax polling for the "load new answers" alert that appears whilst you are writing an answer.

Here's a nice tutorial for ASP.NET using jQuery

For the alert when you attempt to navigate away from a page when you have started typing an answer/ editing a post, look at the onunload event. A confirm dialog is used as opposed to an alert

Answer 2

Are we talking about SO warning you when you try to navigate away from a page with a partially filled in comment/answer ? Check out the onunload() method in Javascript. See here for an example.

Answer 3

add this script to the head portion of your page:

<script language="javascript">
  window.addEventListener('beforeunload', onUnload, false);
  function onUnload(e)
  {
    e.returnValue = "some Text";//this is your message
    e.preventDefault();
  }
</script>