Integrating the Webs' dependency systems

Ian Hickson ian at
Thu May 29 14:31:11 PDT 2014

On Thu, 29 May 2014, Juan Ignacio Dopazo wrote:
> >
> > - some people want to predeclare a bunch of scripts and their 
> >   dependencies, without actually downloading any of them ahead of time, 
> >   and then later, when they're needed, have the entire tree downloaded 
> >   all at once. 
> At the moment we're doing this by running static analysis tools on the 
> ES6 modules and using the information from the dependency tree to know 
> how to download all of them at once.

Right, it's relatively common for people to do this. Not just for scripts, 
indeed, but also for style sheets and other resources. Ideally the browser 
would take care of this logic for the author. This becomes especially 
difficult when there's multiple leaves in the dependency tree, with a lot 
of shared roots, and you just want the files you need (and in particular, 
you don't want to wait for the files to have been downloaded to discover 
what the next level of dependencies is, since that would add one 
round-trip time per dependency level).

> > - sometimes, even a file that is not downloaded immediately upon being 
> >   referenced by another file might still want to be precached, so that 
> >   when it _is_ needed, it's already locally available
> The Loader object is actually a dictionary, so it acts as its own cache. 
> You can call loader.fetch() yourself without the module being executed 
> in order to cache it.

The idea here is that the browser would be able to determine when it's 
best to preload the file; the author would just want to tell the UA that 
the file is something worth precaching, not actually do it.

> > I'm having difficulty understanding the spec for the instantiate hook. 
> > Does it get called before the fetch hook? Is there some way to 
> > dynamically add dependencies later?
> The lifecycle of a module request is normalize -> locate -> fetch -> 
> translate -> instantiate. Instantiate doesn't actually execute anything 
> until all of the dependencies have gone through the same 5 steps.

Aha, ok.

Is there a way to add dependencies before the "fetch" hook is called?

When is the "fetch" hook called in the current ES6 logic? As soon as 
"locate" provides a URL?

Ian Hickson               U+1047E                )\._.,--....,'``.    fL       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'

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