Typescript service typing doesn't allow addition of custom services

[Bassel17]

Bug report

Required System information

• Strapi version: 4.3.2

Describe the bug

Typescript service typing doesn't allow addition of custom services, when adding a new service such as follows:

import { factories } from '@strapi/strapi'; 

export default factories.createCoreService('api::something.something', ({strapi}) =>  ({
  // Method 1: Creating an entirely custom service
  async exampleService(...args) {
    let response = { okay: true }

    if (response.okay === false) {
      return { response, error: true }
    }

    return response
  },
}));

It throws a typing error as follows:

Expected behavior

Should work and not throw an error

[emahuni]

apparently there must have been a regression, this is no longer working.

I was on Strapi 4.3.6, and now on 4.3.8 and on both versions it didn't work properly.

This removes that error, but then introduces another one

If I change the GenericService type to this it works:

from this:

notice any instead of T as the return type

[emahuni]

even the code in this issue doesn't work

[emahuni]

putting any as the return type completely fixed the issue such that this works without a problem:

export type GenericService = Partial<Service> & {
  [method: string | number | symbol]: <T = any>(...args: any) => any;
};


export default factories.createCoreService('api::something.something', ({ strapi })  => {
  return {
    // service functions or properties here
    async someFunction (a: number): Promise<number[]> {
      return [1, 2, 3];
    },
    
    async someFunction2<T> (a: number): Promise<T[]> {
      return [1 as T];
    },
  };
});

notice the generic example at the end, it still works

[saintego]

@emahuni how did you change GenericService without editing factories in lib? Without it I have error on createCoreService

[emahuni]

I created my own typings file then added that type

export type GenericService = Partial<Service> & {
  [method: string | number | symbol]: <T = any>(...args: any) => any;
};

If you put this directly in the file you're using this it'll still work, putting it into a separate typings file makes it cleaner and reusable.