really simple SNS in javascript

index.js

const SNS = require('./sns');
const sns = new SNS();

function smsSubscriber(data) {
  console.log(`send sms for transaction: ${data.transactionId}`);
  return 'sms';
}

async function emailSubscriber(data) {
  //await fetch('/email');
  console.log(`email sent for transaction: ${data.transactionId}`);
  return 'email'
}

const sms1 = sns.subscribe('match', smsSubscriber);
const email2 = sns.subscribe('match', emailSubscriber);
const email3 = sns.subscribe('match', emailSubscriber);

//sns.unsubscribe('match', id2);
sns.unsubscribe('match', email3);

sns.publish('match', {transactionId: '123'}).then((retval) => {
  console.log('all subscribers notified:', retval);
});

const sms2 = sns.subscribe('match', smsSubscriber);

sns.publish('match', { transactionId: '456' });
//console.log(sns.subscirbes);

sns.js

class SNS {
  constructor() {
    this.topics = {};
  }

  createTopic(topic) {
    return this.topics[topic] = { id: 0, lastPublishedData: null, subscribers: {}};
  }

  subscribe(topic, fn) {
    const t = this.topics[topic] || this.createTopic(topic);
    // recall last published data
    if(t.lastPublishedData) {
      fn(t.lastPublishedData);
    }
    t.subscribers[t.id] = fn;
    return t.id++;
  }

  unsubscribe(topic, id) {
    let t = this.topics[topic];
    if (t) {
      delete t.subscribers[id];
    }
  }

  publish(topic, data) {
    let t = this.topics[topic];
    if (t) {
      t.lastPublishedData = data;
      return Promise.all(Object.values(t.subscribers).map((val) => val(data)));
    }
  }
}

module.exports = SNS;