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> onbeforeunload event parameter and empty return value
RainLover
post Jul 30 2014, 02:40 AM
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Q1: I have seen the following code in most sources:

CODE
window.onbeforeunload = function () {
    return 'You might lose your data!';
};

For example:
  1. beforeunload | onbeforeunload event (Internet Explorer)
  2. onbeforeunload event | beforeunload event JavaScript
  3. javascript - How can I override the OnBeforeUnload dialog and replace it with my own? - Stack Overflow

But on MDN the function has an extra parameter: (e). Why is that, what is the difference, and which is right?


Q2: It seems that any browser has a default confirmation message that you cannot customize. Chrome, for example, displays:

QUOTE
Are you sure you want to leave this page?

Can I content myself to this message and leave the return value empty:

CODE
return '';
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Christian J
post Jul 30 2014, 05:18 AM
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QUOTE(RainLover @ Jul 30 2014, 09:40 AM) *

But on MDN the function has an extra parameter: (e). Why is that, what is the difference, and which is right?

Not sure, but I don't think that it has to be called exactly "e", and in the above case I think it's not even used for anything. See http://quirksmode.org/js/introevents.html#link10 (and further down that page) for usage examples.

When using try/catch statements I recall it's mandatory for the catch statement to work: https://developer.mozilla.org/en-US/docs/We...nts/try...catch unsure.gif
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RainLover
post Jul 30 2014, 08:10 AM
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QUOTE(Christian J @ Jul 30 2014, 05:18 AM) *


Yes, it seems unnecessary.
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pandy
post Jul 30 2014, 09:09 AM
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Where do you find that "e"? I don't see it at MSDN. I know I need new glasses, but I really don't find it. It's not in the sample code, that I'm sure of. blush.gif
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Christian J
post Jul 30 2014, 01:16 PM
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It's at MDN, not MSDN:

CODE
window.onbeforeunload = function(e) {
  return 'Dialog text here.';
};

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pandy
post Jul 30 2014, 01:30 PM
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Yeah, I figured as much, but where on the MSDN page? The only code sample I see is this.

CODE
<!DOCTYPE html>
<html>
<head>
<script>
  function closeIt()
  {
    return "Any string value here forces a dialog box to \n" +
         "appear before closing the window.";
  }
  window.onbeforeunload = closeIt;
</script>
</head>
<body>
  <a href="http://www.microsoft.com">Click here to navigate to
      www.microsoft.com</a>
</body>
</html>
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Christian J
post Jul 30 2014, 02:21 PM
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QUOTE(pandy @ Jul 30 2014, 08:30 PM) *

Yeah, I figured as much, but where on the MSDN page?

pandy... rolleyes.gif
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pandy
post Jul 30 2014, 02:47 PM
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WHAT? angry.gif
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pandy
post Jul 30 2014, 02:48 PM
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Uh-oh! blush.gif
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pandy
post Jul 30 2014, 02:52 PM
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Don't you think it's just to show you can pass a parameter that way? It is odd though since they don't seem to do it for the other events I checked.
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Christian J
post Jul 30 2014, 04:16 PM
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Maybe someone just made a typo, can happen to the best of us. happy.gif
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