Type inference is now worse for `app.derive()` on v1.4

[gabrieljablonski]

What version of Elysia is running?

elysia@1.4.5

What platform is your computer?

Linux 5.15.153.1-microsoft-standard-WSL2 x86_64 x86_64

What environment are you using

1.2.22

Are you using dynamic mode?

No

What steps can reproduce the bug?

When the return type for app.derive() is implicitly mixed with null or undefined, type inference breaks.

This did not happen before v1.4

import type Elysia from "elysia";

export interface AuthData {
  id: number;
}

export const authMiddleware = (app: Elysia) =>
  app
    .derive(async ({ request }) => {
      const apiKey = request.headers.get("x-api-key");
      if (!apiKey) {
        return { auth: null };
      }
      const auth: AuthData = { id: 1 };
      return { auth };
    });

export const authGuard = (app: Elysia) =>
  app.use(authMiddleware).onBeforeHandle(({ auth, set }) => {
    console.log(auth?.id); // Property 'id' does not exist on type 'never'.
    console.log(set.status); // Property 'status' does not exist on type 'never'.
  });

What is the expected behavior?

In the example, the TS error Property 'X' does not exist on type 'never'. should not be present, and the type for auth should be correctly inferred.

What do you see instead?

Property 'X' does not exist on type 'never'. error.

Additional information

I went over the changelogs for v1.4, and couldn't tell if this is a expected change, but I don't think that's the case. Please let me know.

Current workaround is to define the return type explicitly on app.derive():

import type Elysia from "elysia";

export interface AuthData {
  id: number;
}

export const authMiddleware = (app: Elysia) =>
  app
    // Explicit return type
    .derive(async ({ request }): Promise<{ auth: AuthData | null }> => {
      const apiKey = request.headers.get("x-api-key");
      if (!apiKey) {
        return { auth: null };
      }
      const auth: AuthData = { id: 1 };
      return { auth };
    });

export const authGuard = (app: Elysia) =>
  app.use(authMiddleware).onBeforeHandle(({ auth, set }) => {
    console.log(auth?.id);
    console.log(set.status);
  });

Have you try removing the node_modules and bun.lockb and try again yet?

Yes.

[SaltyAom]

Should have been fixed in 5a874f1 in #1411

Please kindly see if this works by temporary install the patch version

bun add https://pkg.pr.new/elysiajs/elysia@1411

[gabrieljablonski]

I couldn't manage to install the patch version on my local env (not sure why but whatever 🤷), but testing on StackBlitz, seems to have fixed it.

https://stackblitz.com/edit/vitejs-vite-sncmayvf?file=src%2Ftyping-test.ts

Thanks for checking so quickly!

[lycpan233]

This example will also have the same problem.

import type Elysia from 'elysia';

export interface AuthData {
  id: number;
}

export const authMiddleware = (app: Elysia) =>
  app
    .derive(async ({ request }) => {
      const apiKey = request.headers.get('x-api-key');
      if (!apiKey) {
        return { auth: null };
      }
      const auth: AuthData = { id: 1 };
      return { auth };
    })
    .guard({
      beforeHandle({ auth }) { //  Property 'auth' does not exist on type xxxx
        console.log(auth);
      },
    });