Modules and dependencies known before the load

Ian Hickson ian at hixie.ch
Wed Aug 13 14:59:38 PDT 2014


One of the problems I'm running into when it comes to trying to integrate 
ES6 modules with HTML and new HTML-based dependency features is the way 
that I can't tell ES about dependencies I know about before the data is 
actually fetched and instantiated. The problem can essentially be 
summarised by this line from the definition of ProcessLoadDependencies():

# The ProcessLoadDependencies abstract operation is called after one 
# module has nearly finished loading. It starts new loads as needed to 
# load the module's dependencies.

Suppose a page has this markup:

   <script type=module id=jquery href="jquery.js" whenneeded></script>
   <script type=module id=a href="a.js" uses="jquery" whenneeded></script>
   <script type=module>
    import "a";
    // ...
   </script>

...where "uses" is some hypothetical markup for telling the browser about 
dependencies ahead of time (the assumption being that anims.js contains 
a line like 'import "jquery";'), and where "whenneeded" is some attribute 
that tells the browser to not bother starting the whole loading process 
until the resource is needed.

In this example, the first two <script>s do nothing at first. Then the 
third one is parsed, PromiseOfStartLoadPartwayThrough() is called with the 
contents of the element as the source, and eventually the ES system learns 
that it wants module "a".

Here, the "normalize" and "locate" hooks work together to determien that 
the element with id=a is what we're looking for. (Maybe it should be 
import "#a", to distinguish a package name from an ID, but that's a topic 
for another e-mail.)

At this point, I want to tell the ES6 module that:

  (a) we need to set off a load for that second <script> element, and
  (b) once we have that <script> element's file, it's probably going to
      want to import "jquery", and therefore, we should also set off a
      load for that first <script> element with id=jquery.

Right now, I don't see any way to do (b). ProcessLoadDependencies() is 
called after "instantiate" is done (by InstantiateSucceeded()), and it is 
the first time the ES6 module system tries to load anything.

Ideally I think we should adjust the ES6 module system to support loading 
and compiling code (though not necessarily executing it) for dependencies 
at or around the "fetch" hook.

Failing that, I guess we can also just do that at the HTML level. Will 
that just work? I'm not able to follow the ES spec closely enough to 
determine if, when ProcessLoadDependencies() is called for "a" and finds 
it needs "jquery", it will properly link to the as-yet-not-loaded-but- 
already-in-progress-load for "jquery".

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


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