How do I extend the default type of NPM packages?

Question

We can use the "declare module" to extend the type of module.

eg:

// typings/node-fetch.d.ts
import 'node-fetch';
declare module 'node-fetch' {
    function kk(): void;
}

// src/index.ts
import fetch, { kk } from 'node-fetch';
kk(); // ok

But this is an extension of functionality.

Now I want to extend fetch itself.

I want to have this effect:

// src/index.ts
import fetch from 'node-fetch';
fetch.foo();

The declaration file for node-fetch itself looks like this:

// node_modules/@types/node-fetch/index.d.ts
declare function fetch(
    url: RequestInfo,
    init?: RequestInit
): Promise<Response>;

declare namespace fetch {
    function isRedirect(code: number): boolean;
}

export default fetch;

How can I extend the type of fetch?

I tried it. It doesn't work:

// typings/node-fetch.d.ts
import fetch from 'node-fetch';

declare module 'node-fetch' {
    namespace fetch {
        function foo(): void;
    }
    export default fetch; // Error, Exports and export assignments are not permitted in module augmentations
}

How would you like to solve it, please.

Thanks!

Answer 1

I believe what you need to do is use the interface keyword as shown here: https://stackoverflow.com/a/56861261/106625 –