Convert ember-testing and @ember/test to TS

[wagenet]

No description provided.

[chriskrycho]

I poked at why this is the way it is, and I believe if we update how buildInitializerMethod is defined, we can drop all the casting shenanigans. 😅

// Two significant changes here:
//
// 1. Only parameterize over the bucket name, and simply *infer* the type of
//    initializer from it using the bucket name.
// 2. Make the return type explicit, so that the type parameterization can fall
//    out from it naturally. that the actual returned function can be simpler,
//    not having to do the type parameterization on it.
function buildInitializerMethod<
  B extends 'initializers' | 'instanceInitializers'
>(
  bucketName: B,
  humanName: string
): (this: typeof Engine, initializer: Initializer<Kind<B>>) => void {
  return function (initializer) {
    // ...
  };
}

type Kind<B extends 'initializers' | 'instanceInitializers'> =
  B extends 'initializers' ? Engine : EngineInstance;

Then the usage here will be:

  static initializer = buildInitializerMethod('initializers', 'initializer');

  static instanceInitializer = buildInitializerMethod(
    'instanceInitializers',
    'instance initializer'
  );

playground

[wagenet]

Seems likely. I didn’t try very hard beyond getting it to work, but it does seem worth trying to clean it up a bit.

[wagenet]

So this works except for one significant problem: I think we want initializers on Application to return Application/ApplicationInstance not Engine/EngineInstance.

[chriskrycho]

Gotcha. Hmm. 🤔 That’s… actually impossible to guarantee from a types POV given the existing API, right?

[chriskrycho]

For posterity: We spent some time poking at this and it is indeed impossible to guarantee the correct behavior here… because, and only because, it requires inference on the this type, and that interacts badly with Object.prototype.call. 😩 Net, we'll leave it as is and… hope for a day when initializers are dead?

[chriskrycho]

Per comments above, we should update this to be smarter about how it does its thing.