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I have just been helped on a problem I have here.

var win = window.open(url, name);
win.onunload = StartLoad;
win.close();

To solve this problem pletely, I wanted to know if onunload will be triggered once or every time a event occurs?

In other words, will my function startLoad run every time the child window "win" gets redirected, closed etc? Or will it do this event once and that's it?

Apologies, if this is a silly question.

Thanks all

I have just been helped on a problem I have here.

var win = window.open(url, name);
win.onunload = StartLoad;
win.close();

To solve this problem pletely, I wanted to know if onunload will be triggered once or every time a event occurs?

In other words, will my function startLoad run every time the child window "win" gets redirected, closed etc? Or will it do this event once and that's it?

Apologies, if this is a silly question.

Thanks all

  • javascript
  • events
Share Improve this question edited May 23, 2017 at 12:26 CommunityBot 11 silver badge asked Jul 12, 2009 at 17:36 AbsAbs 58k103 gold badges282 silver badges418 bronze badges
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4 Answers 4

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No - this method can fire multiple times as you navigate off a page in IE6 and IE7.

This code snippet illustrates this (save as OnUnloadTest.htm):

<body>
    <form id="form" action="OnUnloadTest.htm" method="post">
        Click <a href="javascript:form.submit()">here</a>
    </form>
    <script type="text/javascript">
        window.onbeforeunload = beforeunload
        function beforeunload() {
            alert('OnUnload');
        }
    </script>
</body>

Basically, the event fires once for the actual anchor click, and once as the page actually posts back. I've only seen this issue when you have javascript in the href of the anchor, although if you use ASP.NET linkbuttons then be warned as this puts javascript in the href.

For most other sorts of navigation (e.g. user clicks a normal anchor, or closes the browser, or navigates away with a bookmark, etc) the event does only fire once.

It should only fire once, the first time the window unloads. Anything else would be a security hole.

If you want to make sure that your event handler only runs once you can have the handler unbind itself the first time it is invoked. This will guarantee that the callback does not run more than once:

var win = window.open(url, name);
win.onunload = function(event) {
    win.onunload = function() {}; // assign a noop
    return Startload.call(this, event);
};
win.close();

Some JavaScript libraries have a built-in helper for binding an event handler that you only want run once. For example, jQuery has a one() method for this purpose:

var win = window.open(url, name);
$(win).one('unload', Startload);
win.close();

Read WebKit Page Cache II – The unload Event for interesting discussion on how unload event plays with page caching feature of modern browsers.

本文标签： javascriptHow many times is onunload triggeredStack Overflow

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