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Using SCRIPT’s defer Attribute

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One of the seldom used attributes within the HTML tag library is the defer attribute on SCRIPT elements.  As you can probably tell by the name of the attribute, defer instructs the contents of the script tag to not execute until the page has loaded.  Take a look!

Deferring Your Scripts

<script>
	//do stuff (runs first)
</script>
<script defer="defer">
	//do stuff, but defer it  (runs last)
</script>
<script>
	//do more stuff (runs second)
</script>

The deferred SCRIPT element's code will execute once the rest of the page's resources have loaded.  What does this mean?  Be sure that your document doesn't rely on any of the code within the script during page load.  In the example above, the middle block will execute once the page has loaded even though it appears before the last block.

More Details

Olivier Rochard has written an outstanding (and more detailed) post about using the defer attribute on the Mozilla Hacks blog.  His post details browser support (and quality of browser support...or lack thereof), advanced examples, and tips for using the defer attribute.

What I find funny about this tag is that it seems as though most of the script I see should be using this attribute.

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Discussion

  1. It’s seldom used because according to the blog post you linked to the only browser that executed the defer tag correctly is FireFox. IE and Safari have erratic behavior.

    Then one of the last comments on that post points to a bug report that says that HTML5 requires defer to be ignored https://bugzilla.mozilla.org/show_bug.cgi?id=518104

    This seems like a useless tag now.

  2. There is another good article on this over at http://dean.edwards.name/weblog/2005/09/busted/

    Beware of defer, it can be useful if used right, but it is easily used incorrectly

  3. I don’t think “defer” is reliable enough to be used effectively and to think that this means you’ll have better page load performance across the board. Also, the quirks in it can lead to some crazy-to-track-down race conditions.

    Dynamic script loaders are, IMHO, superior to “defer” (or even HTML5’s “async”), because if done right they provide a reliable and normalized behavior across all browsers.

    Some good examples would be LABjs, RequireJS and Dominoes.

    Also, many of the frameworks have script loaders that work well.

  4. kolin

    how would this fit in with ‘domready’ and ‘load’ ?
    i usually run scripts ‘domready’ or ‘load’ depending on whether the browser is trident. could defer replace both?

  5. Wow, I didn’t know you could actually use this outside of IE. Though the behavior seems so unpredictable with certain browsers, it kinda scares me to use it.

  6. Pranav Dave

    Using “defer” has resolved my issue of “Operation Abort” in IE6 and IE7.. Its good to use it always.. as it will not allow anything to write in DOM untill the page is completely loaded….

    • serhio

      not always. if your script does a document.write is harmfull

  7. Javascript Training

    The defer attribute is not well supported in all major browsers. So it should not be used.

  8. Javascript Training

    The defer attribute is not well supported in all major browsers. So it should not be used.

    http://wisentechnologies.com/it-courses/html-css-javascript-jquery-training.aspx

  9. I’m afraid the “defer” attribute is useless.

  10. serhio

    Note: The defer attribute is only for external scripts (should only be used if the src attribute is present).

    So I don’t see the meaning of using it as in the example….

    http://www.w3schools.com/tags/att_script_defer.asp

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