What is the simplest way to have a self-contained event emitter on a TypeScript object?

Question

Given a need for a class to emit multi-listener events with typed payloads, and the following restrictions:

• No ability to inherit a different class, like what the Node.js EventEmitter requires (the class in question already extends something, and there's no possibility to create a mixin in ES6)
• A need for encapsulation, in the way that only if the listener has a direct reference to the object, can it listen to its events (i.e. no global access to listen to the event, like what using standard JS/TS CustomEvent would result in)
• Ability to listen to an event per-object (i.e. only on a specific instance of the class, not based on an event's string identifier - again the only thing that using CustomEvent would allow)

, what is the best approach/package/API to use?

Essentially, what I am looking for is something like this:

/** There is a class called ProcessDoer, which does some kind of work,
 and emits an event once in a while, when the process is done 
(in this example it's just an interval for simplicity).
It has a member variable onProcessDone, which is an event, 
the class of which I would either want to write, 
or if something like this exists in an npm package, know its name. */

class BaseClass {
    public someProperty: number;
}

class ProcessDoer extends BaseClass {
    constructor() {
        super();

        setInterval(() => {
            this.someProperty = Math.random();
            this.onProcessDone.emit(this.someProperty);
        }, 1000);
    }

    public onProcessDone: SomeTypeOfEventEmitter<{ result: number }>;
}

const processDoer1 = new ProcessDoer();
const processDoer2 = new ProcessDoer();

/** In the example, the non-existent `SomeEventEmitterType` has a very
similar structure to `CustomEvent`, i.e. it's declared in a templated way,
with a specific payload variable name and type.
However, the callback isn't registered in a global way, like
`addEventListener`, but specifically on the exact instance needed. */
processDoer2.onProcessDone.addCallback((result: number) => {
    console.log("The second ProcessDoer finished process with result", result);
});