Simple observer pattern with example

index.js

const observer = new Observer();

// Returns a function which, when called, unsubscribes from the observer.
const unsubscribe = observer.subscribe(data => {
  console.log("SingleSubscribe", data);
});

const subscriberA = data => console.log("A", data);
const subscriberB = data => console.log("B", data);

// Returns an array of unsubscribe functions
observer.subscribe([subscriberA, subscriberB]);

observer.set(10);

// To unsubscribe the observer
// unsubscribe()

Observer.js

class Observer {
  constructor(state) {
    this.state = state;

    this.subscribers = [];
  }

  get() {
    return this.state;
  }

  set(state) {
    this.state = state;

    this.broadcast(state);
  }

  subscribeMany(fnArray = []) {
    return fnArray.map(fn => this.subscribe(fn));
  }

  subscribe(fn) {
    if (Array.isArray(fn)) {
      return this.subscribeMany(fn);
    }

    this.subscribers.push(fn);

    return () =>
      (this.subscribers = this.subscribers.filter(
        subscriber => subscriber !== fn
      ));
  }

  broadcast(data) {
    this.subscribers.forEach(subscriber => subscriber(data));

    return data;
  }
}