Helper type to extract the type of a handler parameter with applied plugins

[m1212e]

I needed to export the type of a derive result of a plugin I wrote. It injects a very complex and implicitly typed permission object to the context. I'm not sure if there is a more elegant way to pull this off but I wrote a quick generic TS helper type which you can use to extract the TS type of a context of an elysia instance type at any given point in your api setup:

type UnionToTuple<U, T extends any[] = []> = [U] extends [never]
  ? T
  : (U extends any ? (u: U) => void : never) extends (u: infer I) => void
    ? UnionToTuple<Exclude<U, I>, [...T, I]>
    : never;

// @ts-expect-error TS complains here but we really don't care
type ExtractByIndex<U, I extends number> = UnionToTuple<U>[I];

export type ExtractPluginParamType<
  Instance,
  Method extends "get" | "post" | "put" | "delete" | "patch" | "all" = "post",
> = Parameters<
  // @ts-expect-error TS complains here but we really don't care
  ExtractByIndex<
    // @ts-expect-error TS complains here but we really don't care
    Exclude<Parameters<Instance[Method]>[1], string | number | boolean>,
    0
  >
>[0];

usage looks like this:

// eslint-disable-next-line @typescript-eslint/no-unused-vars
const _typeDummy = new Elysia().use(permissionsPlugin); // apply the plugin we want to be recognized in the type, in our case it uses `.derive(...)` to add a `permissions` field to the context

export type PermissionsType = ExtractPluginParamType<
  typeof _typeDummy // pass the type of our Elysia instance with all required plugins applied
>["permissions"]; // select which field of the context you would like to have extracted via the `["permissions"]`

Obviously you could also access other fields of the context which are non custom and more common like ["cookie"] or ["body"] or whatever you need!

Let me know if this helps you out or if you find a better and more non hacky way of achieving this.