Cannot extend the Strapi interface anymore

[luke-z]

Bug report

Required System information

• Node.js version: 18.15.0
• yarn version: 1.22.19
• Strapi version: 4.11.5
• Database: sqlite
• Operating system: Ubuntu WSL on Windows 11
• Is your project Javascript or Typescript: Typescript

Describe the bug

I'm currently trying to extend the Strapi interface with custom keys in typescript.
I've seen that this feature has been implemented with pr #13108 but it doesn't seem to work anymore.

Steps to reproduce the behavior

1. Create a new project with typescript
2. Add the index.d.ts as mentioned in the pr

import '@strapi/strapi'

declare module '@strapi/strapi' {
    interface StrapiInterface {
        customAddMethod(n1: number, n2: number): number;
    }
}

4. Set and use the custom method in the src/index.ts file

export default {
  register({ strapi }: { strapi: Strapi }) {
    // No type error when trying the access & set the `customAddMethod` method
    strapi.customAddMethod = (n1, n2) => {
      return n1 + n2;
    };
  },

  bootstrap({ strapi }: { strapi: Strapi }) {
    const result = strapi.customAddMethod(11, 31);

    // This should print "42"
    console.log(result);
  },
};

When not importing the Strapi type, the error is: "Cannot use namespace 'Strapi' as a type."
When importing it via import { Strapi } from '@strapi/strapi, the error is "Property 'customAddMethod' does not exist on type 'Strapi'"

Expected behavior

The newly added function should be available on the Strapi interface and not cause any typing problems.

Am I doing something wrong or is this currently not working anymore?
If it's currently not possible, is there an ETA for when it could be possible?

[luke-z]

I managed to find a solution for the issue.

Change the before mentioned index.d.ts from

import '@strapi/strapi'

declare module '@strapi/strapi' {
    interface StrapiInterface {
        customAddMethod(n1: number, n2: number): number;
    }
}

to

import "@strapi/strapi";

declare module "@strapi/strapi" {
  interface Strapi {
    customAddMethod(n1: number, n2: number): number;
  }
}

I don't know since when this works but I feel like I've tried this approach a few months ago without success :P

[strapi-bot]

This issue has been mentioned on Strapi Community Forum. There might be relevant details there:

https://forum.strapi.io/t/how-can-i-extend-some-typescript-types-interfaces-in-strapi-v5/48300/1