How to append type definition to existing @types package?

For example, my project has a dependency on npm package foo-js, and dev-dependency @types/foo-js for type definitions.

// package.json
{
   ...
  "dependencies": {
    "foo-js": "2.0.0"          // latest
  },
  "devDependencies": {
    "@types/foo-js": "1.2.3"   // latest
  }
}

The version of @types/foo-js is outdated (though it is the latest), If I try to use newly added method of foo-js;

import { Foo } from 'foo-js'



Foo.v1_method()     // ok

Foo.v2_new_method() // Property 'v2_new_method' does not exist on type 'typeof import("foo-js")'.ts(2339)

Using type assertion such as (Foo as any).v2_new_method() solves compile-time eror, but I'm trying not to use type assertions as much as possible.

I've tried to add type declaration file as below;

// types.d.ts

declare module 'foo-js' {
  interface Foo {
    v2_new_method: () => any
  }
}

but it didn't work. I think I've missed something.

Is there another way to solve this problem?