[whatwg] Deferring javascript download and execution until after onload

Andy Davies dajdavies at gmail.com
Wed Sep 19 01:14:33 PDT 2012

On 18 September 2012 07:54, Jonas Sicking <jonas at sicking.cc> wrote:
> There has been discussions about implementing a "noexec" attribute on
> <script> which would just load the script but instead of evaluating it
> as soon as it's available, simply fire the "load" event. There would
> also be an additional .execute() function on the script which would
> permit executing the script after it has been loaded.
> This has mostly been discussed in a performance context where people
> has expressed interest in loading scripts, while only waiting to
> execute it until needed in order to avoid the costs involved in
> parsing and executing the script.
> So far Hixie hasn't been interested in putting this into the spec
> though, due to, as I understand it, not being convinced that it's
> needed from a performance point of view.
> I'd personally be interested to see it implemented since people
> clearly do load scripts dynamically and it would seem common for such
> scripts to have dependencies on each other, which means that you have
> to be able to execute them in a certain order.

Thanks for the details...

>From a performance point of view I'm encouraging my clients (and
others are too) to load third-party content asynchronously and where
possible after onload.

I've seen big variations in download times from third-parties like
Facebook and Twitter (measured using synthetic monitoring that
generates waterfalls for each page load) and of course these affect
when the onLoad event fires.



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