Generic types with HelperLike

[swastik]

Thanks for your work on this! 😄 I'm trying to type some external addons that provide helpers. For example:

import { HelperLike } from '@glint/template';

declare module '@glint/environment-ember-loose/registry' {
  export default interface Registry {
    or: HelperLike<{
      Args: { Positional: [any, any] };
      Return: any;
    }>;
  }
}

The helper can be invoked like this and it returns either of the two arguments (for example):

{{#let (or a b) as |foo|}}
  {{foo}}
{{/let}}

However, this seems like it will return an incorrect type: any. Is there a way to type these correctly? Thanks for any help!

[chriskrycho]

Good question! I would write this roughly like this (off the top of my head, so I cannot promise this is exactly right, but it's directionally correct):

declare function or<T extends any[]>(...params: T): T[number];

declare module '@glint/environment-ember-loose/registry' {
  export default interface Registry {
    or: typeof or;
  }
}

Or, with a class-based helper:

import type Helper from '@ember/component/helper';

declare class OrHelper<T extends any[]> extends Helper<{
  Args: {
    Positional: T;
  };
  Return: T[number];
}> {
  compute(positional: T): T[number];
}

declare module '@glint/environment-ember-loose/registry' {
  export default interface Registry {
    or: typeof OrHelper;
  }
}

The use of the T extends any[], ...params: T, and T[number] is to use TypeScript's inference over tuple spread types so that what you're saying is: the positional params are some list of items, and the resulting type is the type of one of those items. You can see that working in this playground.

[swastik]

Excellent, thank you! I verified that works! 🙇🏼