Typescript version of a simple Observable. This was copied from https://stackoverflow.com/a/62002044/592760, translated to TypeScript, and improved.

vanilla-observable.ts

/**
 * Modified by Russell Dempsey on 2021 DEC 15
 * @see https://stackoverflow.com/a/62002044/592760
 */
type Subscriber<T> = (value: T) => void;
class Observable<T> {
    private subscribers = new Set<Subscriber<T>>();

    constructor(private value: T) {}

    get(): T {
        return this.value;
    }

    set(newValue: T): void {
        if (this.value === newValue) return;
        this.value = newValue;

        this.subscribers.forEach((listener) => listener(this.value));
    }

    subscribe(subscriber: Subscriber<T>): () => void {
        this.subscribers.add(subscriber);

        return () => this.unsubscribe(subscriber); // will be used inside React.useEffect
    }

    unsubscribe(subscriber: Subscriber<T>): void {
        this.subscribers.delete(subscriber);
    }
}

export type { Subscriber };
export { Observable };

https://stackoverflow.com/a/62002044