having this some.js file

export function myFunc(a, b) {
  return a.asdf + b;
}

I want to be able to write something like this let’s say in some.d.ts file

export declare function myFunc(a: number, b: number): number;

then run ‘npx tsc’ command which will give me ‘asdf is not a member of type number’,
but right now i get errors saying “parameter a implicitly has any type”.
I have this tsconfig.json

{
  "compilerOptions": {
    "allowJs": true,
    "checkJs": true,
    "noEmit": true,
    "strict": true,
    "skipLibCheck": false,
  },
  "include": ["**/*.js", "**/*.d.ts"]
}

Is it that I miss some ts options or I should declare types in .d.ts differently? If it is impossible, what is the general way to program in js but having typings and not in one global.d.ts file?

I also tried

declare namespace some {
  export function myFunc(a: number, b: number): number;
}

and

declare module "some" {
  export function myFunc(a: number, b: number): number;
}