gistfile1.js

'use strict';

class Observable {
  constructor(subscribe) {
    this.observers = [];
    if (subscribe) {
        subscribe(this)
    }
  }

  subscribe(observer) {
    const index = this.observers.push(observer);
    const del = () => {
        this.observers.splice(index -1, 1)
    }
    return {
        ...this,
        unsubscribe() {
            del()
        }
    };
  }

  next(data) {
    if (this.observers.length === 0) return;
    for (const observer of this.observers) {
      observer(data);
    }
  }
}

const randomChar = () => String
  .fromCharCode(Math.floor((Math.random() * 25) + 97));

// Usage
const sourceStream = new Observable(subscriber => {
    setInterval(() => {
        const char = randomChar();
        subscriber.next(char);
    }, 200);
});

let count = 0;
const observer = (char) => {
  process.stdout.write(`${char}\n`);
  count++;
  if (count > 50) {
    process.stdout.write('\n\n');
    process.exit(0);
  }
};

const subscription = sourceStream.subscribe(observer);

sourceStream.subscribe(observer)

setTimeout(() => {
    subscription.unsubscribe()
}, 1000)