This may have been simpler than I thought. The "load randomly" workaround seems to be just fine:

mw.loader.using( 'ext.cite.referencePreviews' )

I can include an error handler as the third parameter, and find some way of skipping the dependent tests.

There's no built-in mechanism in ResourceLoader, but it's fairly easy to implement it for your own module, for example:

• https://gerrit.wikimedia.org/g/mediawiki/extensions/DiscussionTools/+/c5044b08223d9676578ff96067aae856dda7fa6d/extension.json#269
• https://gerrit.wikimedia.org/g/mediawiki/extensions/DiscussionTools/+/c5044b08223d9676578ff96067aae856dda7fa6d/includes/ResourceLoaderData.php#120

(This would work when defining the QUnitTestModule as well.)

In T359409#9620961, @matmarex wrote:

There's no built-in mechanism in ResourceLoader, but it's fairly easy to implement it for your own module, for example:

Elegant workaround! I've used this now to implement an optional dependency for the QUnitModule and it helps with the situation, by causing the depdency to be conditionally required and made available.

However, there's still a gap caused by the inability to conditionally skip QUnit tests. In case it clarifies my issue, I'm currently forced to use code like this in each test:

( mw.loader.getModuleNames().indexOf( 'ext.popups.main' ) !== -1 ?
       QUnit.module :
       QUnit.module.skip )( 'ext.cite.referencePreviews#createReferenceGateway', {

I could use the same trick as you suggest in order to dynamically change the tests so that units dependent on Popups are absent, but I prefer to make the skipping visible. Ideally, QUnit would support something like before() { if ( COND ) this.skipModule() } but does not.

@awight

• You can group these tests together in a nested QUnit.module scope, and skip it in its entirety using a single check by calling QUnit.module.skip. https://api.qunitjs.com/QUnit/module/

QUnit.module( 'ext.Example', function () {

  QUnit.test( 'a', … ); // ext.Example:  a
  QUnit.test( 'b', … ); // ext.Example: b

  QUnit.module.skip( 'popups', function () {

    QUnit.test( 'c', … ); // SKIP ext.Example: popups: c
    QUnit.test( 'd', … ); // SKIP ext.Example: popups: d

  } );
} );

• You can use mw.loader.getState() to simplify that check. https://doc.wikimedia.org/mediawiki-core/master/js/mw.loader.html#.getState

That makes sense, but my question is how to do this dynamically? The only workaround I've come up with is quite an eyesore: T359409#9623218 . Maybe it will look nicer if I assign the module contents to a variable and write the conditional something like this:

const moduleBody = function () ...

if ( POPUPS_COND ) {
  QUnit.module( 'cite.and.popups', moduleBody );
} else {
  QUnit.module.skip( 'cite.and.popups', moduleBody );
}

I guess what I'm looking for is something more dynamic which can set the module skip property from e.g. a before hook. But looking at QUnit source code, I don't think this can be done.

You could wrap that in a function, right?

QUnit.moduleSkipIf = function ( name, cond, body ) {
  ( !cond ? QUnit.module : QUnit.module.skip )( name, body );
};
QUnit.moduleSkipIf( 'cite.and.popups', mw.loader.getState( 'ext.popups.main' )..., ... )

I'm not sure what the problem is at this point. Even without that, it seems clear enough for me. I don't feel like adding a helper in core would be helpful here, at least not until we see this pattern in several extensions.

Note that the entire file does need to be wrapped by the skip per-se. Something like following could work as well.

if ( POPUPS_COND ) {
  QUnit.test.skip( 'cite.and.popups' );
  return;
}

QUnit.module( 'cite.and.popups', function ( hooks ) {
  QUnit.test( …, function ( assert ) {
    …
  } );
} );

QUnit.test.skip() does not require a function body. One could also leave out the skip call. But I suppose it's useful to at least include a trace in the CLI output that a test was skipped. That's essentially all it is. A way to print a message.