class BSubject {
  private value: any;
  private readonly subscribers: Function[] = [];
  constructor(value: any) {
    this.value = value;
  }
  subscribe(f: Function) {
    this.subscribers.push(f);
    f(this.value);
  }
  next(value: any) {
    this.value = value;
    this.subscribers.forEach(f => f(this.value));
  }
}

let i = 0;
const sub = new BSubject(i);

setInterval(() => sub.next(++i), 1000);

sub.subscribe((value: any) => console.log(`sub1: ${value}`));
setTimeout(() => sub.subscribe((value: any) => console.log(`sub2: ${value}`)), 2000);
setTimeout(() => sub.subscribe((value: any) => console.log(`sub3: ${value}`)), 4000);