This is my experiment rewriting a part of rxjs lib with pure js, that provide same effect as the original one

    I start with the Observable class, to check to original class, pls check the doc https://rxjs.dev/guide/observable

    Check this sandbox for my implementation https://codesandbox.io/s/rxjs-rewrite-observable-v6jqd?file=/src/App.js

    A. Observable and subscription

    First of all, we create a simple rxjs original observable

const numberObservable = new Observable((subscriber) => {
  subscriber.next(1);
  setTimeout(() => {
    subscriber.next(2);
    subscriber.complete();
    subscriber.next(3);
  }, 5000);
});

    and a simple react view to show the subscription result

export default function App() {
  const [messages, setMessages] = useState([]);
  const consumer = {
    next(x) { setMessages((messages) => [...messages, x]); },
    error(x) { setMessages((messages) => [...messages, `error: ${x}`]); },
    complete() { setMessages((messages) => [...messages, "done"]); }
  };
  useEffect(() => {
    const numberSubscription = numberObservable.subscribe({ ...consumer });
  }, []);
  return (
    <div className="App">
      {messages.map((i) => (
        <p>{i}</p>
      ))}
    </div>
  );
}

    Now the result should be 1 => 2 => done (3 will not shows, cause subscriber.next(3) command executed after complete / error one )

    So start to create my own Observable class (MyObservable)

class MyObservable {
  constructor(handler) {
  }
  subscribe(subscriberFn) {
  }
}

    First step is implementing the constructor. The original Observable class take 1 function as argument. So in MyObservable class constructor, a handler is created to save that function. The handler has a (bool)isStopped flag to indicate that it's complete/error or still active

  constructor(handler) {
    this.handler = handler;
    this.handler.isStopped = false;
  }

    Next step is implementing the subscribe method. The original subscribe method take a consumer (which is just an object include 3 pure functions: next, complete, error) as argument. So we have to modify those 3 a bit to make sure the next function can only be executed if it's not completed or got an error. When an error/complete command executed, isStopped flag update to true, then after that, omit the next function execution

  subscribe(subscriberFn) {
    const err = subscriberFn.error;
    const comp = subscriberFn.complete;
    const n = subscriberFn.next;
    subscriberFn.error = (value) => {
      if (!this.handler.isStopped) {
        this.handler.isStopped = true;
        err(value);
      }
    };
    subscriberFn.complete = () => {
      if (!this.handler.isStopped) {
        this.handler.isStopped = true;
        comp();
      }
    };
    subscriberFn.next = (value) => {
      if (!this.handler.isStopped) {
        n(value);
      }
    };
    this.handler(subscriberFn);
  }

    Finally, replace the original Observable by MyObservable. You should see the same result.

    B. Unsubscription.

    Next part is unsubscription. Pls check the orginal doc https://rxjs.dev/guide/subscription From the doc, we could modify the original observable above this

const numberObservable = new MyObservable((subscriber) => {
  ...
  return () => {
    console.log("logic unsubscribe number");
  };
});

    Then if we modify the react view like this:

useEffect(() => {
    const numberSubscription = numberObservable.subscribe({ ...consumer });
    setTimeout(() => {
          numberSubscription.unsubscribe();
    }, 2000);
  }, []);

    We should see the output logic unsubscribe number printed after 2 sec, at console.log tab. Note that, Subscriptions can also be put together, so that a call to an unsubscribe() of one Subscription may unsubscribe multiple Subscriptions. So if we have an other subscription (let say letterSubscription of letterObservable)

const letterObservable = new MyObservable((subscriber) => {
  subscriber.next("A");
  return () => {
    console.log("logic unsubscribe letter");
  };
});

useEffect(() => {
    const numberSubscription = numberObservable.subscribe({ ...consumer });
    setTimeout(() => {
      numberSubscription.unsubscribe();
    }, 2000);
    const letterSubscription = letterObservable.subscribe({ ...consumer });
    numberSubscription.add(letterSubscription);
  }, []);

    We should see the output logic unsubscribe number => logic unsubscribe letter printed after 2 sec, at console.log tab.

    So start to modify MyObservable again

    We have unscribeList list to tore all supscription

constructor() {
    ...
    this.unscribeList = [];
} 

    And then subscribe method in turn, will return an object of 2 functions: 1 for add an subscription, and the other is for unsubscribe all of them.

subscribe(subscriberFn) {
    ...
    const unsubscribeFn = this.handler(subscriberFn);
    return {
      unsubscribe: () => {
        this.handler.isStopped = true;
        unsubscribeFn();
        this.unscribeList.map((fn) => fn.unsubscribe());
      },
      add: (fn) => {
        this.unscribeList.push(fn);
      }
    };
}

    Finally, replace the original Observable by MyObservable. You should see the same result again. Check the sandbox https://codesandbox.io/s/rxjs-rewrite-observable-v6jqd?file=/src/App.js:225-248 for the implementation.