How to extend NodeJS.ReadableStream prototype in TypeScript 2.5?

Question

I'd like to add a prototype function to NodeJS.ReadableStream. Here is my attempted code along with resulting errors.

my.d.ts

declare namespace NodeJS {
    interface ReadableStream {
        readAll(encoding: string): Promise<string>;
    }
}

my.ts

namespace NodeJS {
    ReadableStream.prototype.readAll = function (this: ReadableStream, encoding: string) {
        // do stuff
    };
}

Errors

• lib.es6.d.ts: Property 'readAll' does not exist on type 'ReadableStream'.

  I'm not sure how to properly reference NodeJS ReadableStream interface instead of the lib.es6's ReadableStream interface within my.ts. I tried to provide scoping by means of namespace NodeJS, but it does not work.

• node\@types\index.d.ts: Class 'Readable' incorrectly implements interface 'ReadableStream'. Property 'readAll' is missing in type 'Readable'.

Related

Note, I tried using my learning from How to extend String Prototype and use it next, in TypeScript question, but haven't arrived at a solution.

Edit #1

As requested, here's the code from node\@types\index.d.ts that is producing the second error. The last line where Readable class is defined is specifically the error. The code is sourced from dependency "@types/node": "^8.5.8". Note, I feel this is the pertinent code from the 7000+ line file and won't be pasting the rest. You can view it in full glory here.

declare module "stream" {
    import * as events from "events";

    class internal extends events.EventEmitter {
        pipe<T extends NodeJS.WritableStream>(destination: T, options?: { end?: boolean; }): T;
    }

    namespace internal {
        export class Stream extends internal { }

        export interface ReadableOptions {
            highWaterMark?: number;
            encoding?: string;
            objectMode?: boolean;
            read?: (this: Readable, size?: number) => any;
            destroy?: (error?: Error) => any;
        }

        export class Readable extends Stream implements NodeJS.ReadableStream {

Edit #2

Working through the suggestions in the comments, I've arrived at the following code, which solves both of the original compiler errors. However, another error is popping up at run-time, myReadableStreamVariable.readAll is not a function.

my.ts

declare namespace NodeJS {
    interface ReadableStream {
        readAll: typeof readAll;
    }
}

function readAll(this: NodeJS.ReadableStream, encoding: string) {
    // do stuff
};

hack.ts

import { Readable } from "stream";

declare module "stream" { interface Readable extends NodeJS.ReadableStream { } }