cannot use d.ts file to load declaration merging

works fine perfectly when these are in the same file.

declare module "express-session" {
    interface SessionData {
        userId: number;
    }
}


static async loginUser(req: Request, res: Response) {
    const { email, password } = req.body;
    req.session!.userId = 1;
    return res.send("login");
}

However, if I seperate these files into,

index.d.ts index.ts

then it shows that userId does not exist within SessionData or Partial.

Why is this so?