RxJS Extension Observed Class with Operators

Question

How can I extend a class Observableby applying the built-in RxJS operators to it?

I would like to do something like this:

class TruthyObservable extends Observable {
  constructor(subscriber) {
    super(subscriber);

    return this.filter(x => x);
  }
}

class TruthyMappedObservable extends TruthyObservable {
  constructor(subscriber) {
    super(subscriber);

    return this.map(x => `'${x}'`);
  }
}

Can this be done without returning the constructor?

Answer 1

It pretty much depends on what you want to do, but let me say what you want to do TruthyObservable, which behaves the same as the default Observable.create(...), but only passes even numbers:

import { Observable, Observer, Subscriber, Subject, Subscription } from 'rxjs';
import 'rxjs/add/operator/filter';

class TruthyObservable<T> extends Observable<T> {

    constructor(subscribe?: <R>(this: Observable<T>, subscriber: Subscriber<R>) => any) {
        if (subscribe) {
            let oldSubscribe = subscribe;
            subscribe = (obs: Subscriber<any>) => {
                obs = this.appendOperators(obs);
                return oldSubscribe.call(this, obs);
            };
        }

        super(subscribe);
    }

    private appendOperators(obs: Subscriber<any>) {
        let subject = new Subject();

        subject
            .filter((val: number) => val % 2 == 0)
            .subscribe(obs);

        return new Subscriber(subject);
    }

}

let o = new TruthyObservable<number>((obs: Observer<number>) => {
    obs.next(3);
    obs.next(6);
    obs.next(7);
    obs.next(8);
});

o.subscribe(val => console.log(val));

Sent to the console:

6
8

See demo version: https://jsbin.com/recuto/3/edit?js,console

, Observable _subscribe(), , , ( Observable ). _subscribe() _subscribe, , - , . _subscribe , Subject, filter() appendOperators(). , Observer Subject obs = this.appendOperators(obs).

, , . obs.next(3); Subject, Observer.

Answer 2

, , :

    Observable.prototype.truthy = function truthy() {
        return this.filter(x => x);
    }