Is there a cross-browser onload event when clicking the back button?

Some modern browsers (Firefox, Safari, and Opera, but not Chrome) support the special "back/forward" cache (I'll call it bfcache, which is a term invented by Mozilla), involved when the user navigates Back. Unlike the regular (HTTP) cache, it captures the complete state of the page (including the state of JS, DOM). This allows it to re-load the page quicker and exactly as the user left it.

The load event is not supposed to fire when the page is loaded from this bfcache. For example, if you created your UI in the "load" handler, and the "load" event was fired once on the initial load, and the second time when the page was re-loaded from the bfcache, the page would end up with duplicate UI elements.

This is also why adding the "unload" handler stops the page from being stored in the bfcache (thus making it slower to navigate back to) -- the unload handler could perform clean-up tasks, which could leave the page in unworkable state.

For pages that need to know when they're being navigated away/back to, Firefox 1.5+ and the version of Safari with the fix for bug 28758 support special events called "pageshow" and "pagehide".

References:

  • Webkit: http://webkit.org/blog/516/webkit-page-cache-ii-the-unload-event/
  • Firefox: https://developer.mozilla.org/En/Using_Firefox_1.5_caching.
  • Chrome: https://code.google.com/p/chromium/issues/detail?id=2879

Guys, I found that JQuery has only one effect: the page is reloaded when the back button is pressed. This has nothing to do with "ready".

How does this work? Well, JQuery adds an onunload event listener.

// http://code.jquery.com/jquery-latest.js
jQuery(window).bind("unload", function() { // ...

By default, it does nothing. But somehow this seems to trigger a reload in Safari, Opera and Mozilla -- no matter what the event handler contains.

[edit(Nickolay): here's why it works that way: webkit.org, developer.mozilla.org. Please read those articles (or my summary in a separate answer below) and consider whether you really need to do this and make your page load slower for your users.]

Can't believe it? Try this:

<body onunload=""><!-- This does the trick -->
<script type="text/javascript">
    alert('first load / reload');
    window.onload = function(){alert('onload')};
</script>
<a href="http://stackoverflow.com">click me, then press the back button</a>
</body>

You will see similar results when using JQuery.

You may want to compare to this one without onunload

<body><!-- Will not reload on back button -->
<script type="text/javascript">
    alert('first load / reload');
    window.onload = function(){alert('onload')};
</script>
<a href="http://stackoverflow.com">click me, then press the back button</a>
</body>

Tags:

Javascript

Pageload

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