Defining TypeScript Types for Conditions

[nbibler]

I imagine I'm just missing something in the documentation, but I'm trying to figure out how to add TypeScript types to conditions for a non-SQL-backed, purely POJO application. I assume there's got to be some way to inform MatchConditions what the Subject names to types are?

import { AbilityBuilder, type MatchConditions, PureAbility } from "@casl/ability";

type User = {id: string; blogPostIds: string[] };
type BlogPost = { id: string; title: string };

export type AppAction = "read";
export type AppSubject = BlogPost | "BlogPost";
export type AppAbility = PureAbility<[AppAction, AppSubject], MatchConditions>;
const lambdaMatcher = (matchConditions: MatchConditions) => matchConditions;

export function defineAbility(user: User): AppAbility {
  const { can, build } = new AbilityBuilder<AppAbility>(PureAbility);

  // blogPost = "AnyObject" in TypeScript, here, how do I convince CASL it is a BlogPost type and get property type checks?
  can("read", "BlogPost", (blogPost) => user.blogPostIds.includes(blogPost.id));

  return build({ conditionsMatcher: lambdaMatcher });
}

[nbibler]

The example in the documentation returns TypeScript warnings/errors, likely because it doesn't appear to even attempt to define what an Article is:

import {
  PureAbility,
  AbilityBuilder,
  type AbilityTuple,
  type MatchConditions,
} from "@casl/ability";

type AppAbility = PureAbility<AbilityTuple, MatchConditions>;
const lambdaMatcher = (matchConditions: MatchConditions) => matchConditions;

export default function defineAbilityFor(user: any): AppAbility {
  const { can, build } = new AbilityBuilder<AppAbility>(PureAbility);

  can("read", "Article", ({ authorId }) => authorId === user.id);
  can("read", "Article", ({ status }) =>
    ["draft", "published"].includes(status),
  );

  return build({ conditionsMatcher: lambdaMatcher });
}

[stalniy]

Please read it here https://casl.js.org/v6/en/advanced/typescript#ability-builder-type-inference

[nbibler]

So.. is it as "simple" as:

type BlogPost = { id: string; title: string };

export type AppSubject =
  | BlogPost & { __caslSubjectType__: "BlogPost" }
  | "BlogPost";

export type AppAbility = PureAbility<[AppAction, AppSubject], MatchConditions>;

[nbibler]

Or should that be using a library-provided helper? i.e. TaggedInterface ?

[nbibler]

For anyone curious, it looks like CASL exposes a ForcedSubject generic type that can be used for this which applies the magic __caslSubjectType__ recognition to the types:

import { type ForcedSubject } from "@casl/ability";

type BlogPost = { id: string; title: string };

export type AppSubject =
  | (BlogPost & ForcedSubject<"BlogPost">)
  | "BlogPost";