Based on the original eventBus module described here: https://medium.com/@soffritti.pierfrancesco/create-a-simple-event-bus-in-javascript-8aa0370b3969

eventBus.js

/**
 * subscriptions data format: 
 * { eventType: { id: callback } }
 */
const subscriptions = { };
const getNextUniqueId = getIdGenerator();

function subscribe(eventType, callback) {
    const id = getNextUniqueId();

    if (!subscriptions[eventType]) {
        subscriptions[eventType] = { };
    }

    subscriptions[eventType][id] = callback;

    return { 
        unsubscribe: () => {
            delete subscriptions[eventType][id];
            if (Object.keys(subscriptions[eventType]).length === 0) {
                delete subscriptions[eventType];
            }
        }
    }
}

function publish(eventType, arg) {
    if (!subscriptions[eventType]) {
        return;
    }

    Object.keys(subscriptions[eventType]).forEach(key => subscriptions[eventType][key](arg));
}

function getIdGenerator() {
    let lastId = 0;
    
    return function getNextUniqueId() {
        lastId += 1;
        return lastId;
    }
}

module.exports = { publish, subscribe };

• updated with some semi-colons and brackets