const source = {
  value: 1,
  subscribers: new Set(),
  subscribe(fn) {
    this.subscribers.add(fn);
    return () => this.subscribers.delete(fn);
  },
  next(newValue) {
    this.value = newValue;
    this.subscribers.forEach(fn => fn(this.value));
  }
};

const mapped = {
  value: undefined,
  source,
  transform: x => x * 2,
  init() {
    this.source.subscribe(val => {
      this.value = this.transform(val);
      console.log(`Mapped: ${this.value}`);
    });
  }
};

mapped.init();
source.next(3);
source.next(5);
console.log(`Final: ${mapped.value}`);
source.next(2);