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From "Onno Scheffers (JIRA)" <...@tapestry.apache.org>
Subject [jira] Commented: (TAPESTRY-2364) YSlow Recommendation: Write Scripts at bottom of page
Date Mon, 06 Oct 2008 19:45:44 GMT

    [ https://issues.apache.org/jira/browse/TAPESTRY-2364?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12637199#action_12637199
] 

Onno Scheffers commented on TAPESTRY-2364:
------------------------------------------

> this is simply untrue.
> QUOTE: "Otherwise it is impossible to execute Javascript before the page renders..."

You cut off the part of the sentence that really mattered.

An onload-event is triggered when ALL resources are loaded (including images and script-files).
The page starts rendering as soon as possible (even before the resources are loaded). Have
you never seen the text popup in a browser before the images were loaded? That's the difference
between dom:loaded and the onload event.

This means rendering starts BEFORE the Prototype libraries are even loaded, which means the
custom Javascript will be executed even later, since that relies on the Prototype libraries
being loaded. It really is true. I can even prove it with example-code if you don't believe
me.


> A listener to onload can be added no matter where script tags are. onload will not be
called until all page resources are loaded.

Indeed, but the page will be rendered before the resources are loaded.. you can see the text
appear in the browser before the images are loaded quite clearly.


> you can always make content hidden, and show it after parsing it.

But not before _rendering_ it!

> YSlow Recommendation: Write Scripts at bottom of page
> -----------------------------------------------------
>
>                 Key: TAPESTRY-2364
>                 URL: https://issues.apache.org/jira/browse/TAPESTRY-2364
>             Project: Tapestry
>          Issue Type: Improvement
>          Components: tapestry-core
>            Reporter: Howard M. Lewis Ship
>            Assignee: Howard M. Lewis Ship
>            Priority: Minor
>             Fix For: 5.0.12
>
>
> Tapestry is already doing a good job on this, by writing the dynamically generated <script>
block at the bottom of the page, but the external scripts should also be down there.
> http://developer.yahoo.com/performance/rules.html#js_bottom

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