How do I type something that isn't already built in to the framework?

[NullVoxPopuli]

Per this suggestion: emberjs/rfcs#822 (comment)

I was trying to figure out how to type class-based resources.

after my class definition, I tried the example from the linked comment above:

export interface Resource<T extends ArgsWrapper> extends InstanceType<
  HelperLike<{
    Args: { Named: T['named']; Positional: T['positional'] };
    // this is the instance of the Resource -- which I also don't know how to get
    // Return: IDontKnowHowToGetTheValueTypeOfAResourceButItGoesHere
    Return: Resource<T> // how to make this work with sub classes?
  }>
> {}

now, I'm working with the class definition here because folks can do:

class MyResource extends Resource {
  @tracked foo;
}

<template>
  {{#let (MyResource whateverArgs) as |instance|
    {{instance.foo}}
  {{/let}}
</template>

In JS/TS, I have some type-help via a static method (this is for context, rather than directly related to the issue as resources are usable in both templates and js/ts):

class {
  instance = MyResource.from(() => whateverArgs);
}

but anyway, back to HelperLike, I'm getting this error:

 TS2344: Type 'HelperLike<{ Args: { Named: T["named"]; Positional: T["positional"]; }; Return: new (...args: any) => any; }>' does not satisfy the constraint 'new (...args: any) => any'.
  Cannot assign an abstract constructor type to a non-abstract constructor type.

So, it seems HelperLike cannot be passed to InstanceType.

I can remove the InstanceType and the red squigglies go away, however, I get a very hard to read error in one of the utilities consming this class:

 TS2344: Type 'TrackedArrayMap<Element, MapTo>' does not satisfy the constraint 'Resource<ArgsWrapper>'.
  Type 'Invokable<(args_0: NamedArgs<Element, MapTo>, args_1: Element[]) => Resource<{ positional: PositionalArgs<Element>; named: NamedArgs<Element, MapTo>; }>>' is not assignable to type 'Invokable<(...args: [named: EmptyObject | Record<string, any>, ...positional: unknown[]] | [named: EmptyObject | Record<string, any>]) => Resource<ArgsWrapper>>'.
    Type '(args_0: NamedArgs<Element, MapTo>, args_1: Element[]) => Resource<{ positional: PositionalArgs<Element>; named: NamedArgs<Element, MapTo>; }>' is not assignable to type '(...args: [named: EmptyObject | Record<string, any>, ...positional: unknown[]] | [named: EmptyObject | Record<string, any>]) => Resource<ArgsWrapper>'.
      Types of parameters 'args_0' and 'args' are incompatible.
        Type '[named: EmptyObject | Record<string, any>, ...positional: unknown[]] | [named: EmptyObject | Record<string, any>]' is not assignable to type '[NamedArgs<Element, MapTo>, Element[]]'.
          Type '[named: EmptyObject | Record<string, any>, ...positional: unknown[]]' is not assignable to type '[NamedArgs<Element, MapTo>, Element[]]'.
            Target requires 2 element(s) but source may have fewer.
src/util/map.ts (182:39)

182   let resource = TrackedArrayMap.from<TrackedArrayMap<Element, MapTo>>(
                                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

from is inherited from Resource, which looks like

  static from<Instance extends Resource<ArgsWrapper>>(
    this: (new (...args: unknown[]) => Instance),
    context: object,
    thunk?: Thunk | (() => unknown)
  ): Instance {

🤔 this is a lot, I made a TS playground

So I guess assuming I'm using HelperLike correctly, how do I read this new error? is there anything I could do about my types to flatten it? or to make it less verbose?
I get lost mid-way though.

[dfreeman]

So I guess assuming I'm using HelperLike correctly,

You're not 😉
The snippet I gave you with InstanceType was using it correctly, and linked to an example of that exact usage working in practice in @glint/ember-environment-ember-loose.

What version of TS are you using? InstanceType has accepted abstract constructors since this PR that landed in 4.3. Note that writing extends InstanceType<HelperLike<...>> in the playground you linked doesn't produce an error.

[NullVoxPopuli]

oh! I was using typescript 4.2. Gonna try dropping support for 4.2, 4.3, and 4.4, since all this work I'm doing is in prep for a major anyway 🙃

[NullVoxPopuli]

success!!!
Silly 4.2

thank you!

[NullVoxPopuli]

Well, my error about map remains. but at least InstanceType is working

[NullVoxPopuli]

More specific questions to come later