How to augment TypeScript interface in d.ts

Question

My use case: request's RequestResponse type definition is missing the body property and looks like this:

declare namespace request {

    // ...

    export interface RequestResponse extends http.IncomingMessage {
        request: Options;
    }

    // ...

}
declare var request: request.RequestAPI<request.Request, request.CoreOptions, request.RequiredUriUrl>;
export = request;

I'm trying to fix it by creating a request-fix.d.ts file with something like this:

import * as http from 'http';
declare namespace request {
    export interface RequestResponse extends http.IncomingMessage {
        body: any;
    }
}

But it has no effect. My end goal is that in my app.ts, I can do this:

import * as rp from 'request-promise';
import { RequestResponse } from 'request';

let response = rp.get(url);

response.statusCode; // works
response.body; // doesn't compile

Of course I could just contribute to DefinitelyTyped :) But this question is about to augment the RequestResponse interface.

Answer 1

Here is the combination that works in request-fix.d.ts:

import * as http from 'http';

declare module 'request' {
    export interface RequestResponse extends http.IncomingMessage {
        body: any;
    }
}

To augment existing module, declare module must be used instead of declare namespace, and it must appear in the module scope somewhere among the compiled sources.

That is, request-fix.d.ts must have some import at top level to turn it into a module, as you did with import * as http from 'http' in your code. If declare module appears in the non-module scope, (as I did with the first attempt at the answer), it just declares separate, unrelated module as described here.