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Message-ID: <1672892.tfqf7eurea@PointedEars.de>
From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Reply-To: Thomas 'PointedEars' Lahn <usenet@PointedEars.de>
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Date: Thu, 30 May 2013 05:13:58 +0200
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Subject: Re: Parallel Download Abstraction
Newsgroups: comp.lang.javascript
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Michael Haufe (TNO) wrote:

> Thomas 'PointedEars' Lahn wrote:
>> Michael Haufe (TNO) wrote:
>> > Will the following perform the same parallel download semantics if it
>> > is abstracted into a different file?
>> > 
>> > ...
>> > <script>
>> > document.write("<script src='script1'></script>")
>> > document.write("<script src='script2'></script>")
>> > </script>
>> > <body>
>> > </html>
>> > 
>> > becoming:
>> >
>> > <script src='writeTwo'></script>
>> > </body>
>> > </html>
>>
>> Yes, it will also _not_ achieve “parallel download”.
> 
> Did you say "also", because I forgot to escape the </script> tag for
> this post, or is there another reason as well?

Please do not top-post.

I said “also” because it is unlikely that either of the two approaches will 
result in “parallel download”.  Scripts, when included this way, are always 
processed sequentially, and AFAIK are also downloaded sequentially:

<https://developers.google.com/speed/articles/include-scripts-properly>

Usually it is a bad idea to use document.write() to include content, 
particularly script content, nowadays:

<https://developers.google.com/speed/docs/best-practices/rtt?hl=en#AvoidDocumentWrite>

A possibility for parallel download is asynchronous XHR using two different 
XHR instances, and inserting “script” elements with the resource's content 
each upon completion, as I do synchronously in jsx.importFrom():

<http://PointedEars.de/websvn/filedetails.php?repname=JSX&path=/trunk/object.js>

(Changing that to asynchronous loading is accomplished by simply passing 
“true” instead of “false” for the third argument.  I shall work on that;
to date it did not occur to me that it could be useful to load scripts 
asynchronously.)


I did not comment on the missing escaping of </script> because from your 
<script> tag I assumed that you would use HTML5, and I assumed that because 
it is not SGML-based, escaping of “</” was no longer necessary there.  
However, indeed it is still necessary to avoid </script> being parsed as the 
end-tag of the “script” element:

<http://www.w3.org/TR/2012/CR-html5-20121217/scripting-1.html#restrictions-for-contents-of-script-elements>

That said, for backwards-compatibility it is certainly wise to escape “</” 
*always* within a “script” element.  Likewise for using 
“type="text/javascript"” on “script” elements and “type="text/css"” on 
“style” elements.

-- 
PointedEars