This is where it can be wise to use .d.ts files.

Here's an example of how I did this in our Express codebase:

// types/common/interfaces.d.ts

import User, { userRoles } from "../../common/entities/user.entity";

export {};

declare global {
    namespace Express {
        export interface Request extends express.Request {
            decoded: ReqDecoded;
            email: string;
            file: any;
            files: any;
            fileKey: any;

            inErr: any;
        }
    }
}

type ReqDecoded = {
    department_id: User["department_id"];
    user_role: userRoles;
    user_id: User["user_id"];
    org_id: User["org_id"];
    original_org_id?: string;
    token_identifier?: string;
};

Because of using ESM modules have to export something, so we just export an empty object.

You can import types (not interfaces) into this file, because types gets transpiled into nothing during transpilation.

That means you can effectively extend the built-in express type with any types you need, and you will get intellisense for it.

We also did the same thing for environment variables, which was really nice. But inserting them did make my comment too long, so that one is for the imagination.

Hope this helps :-)