Simple PubSub implementation with JavaScript - taken from Addy Osmani's design patterns book & ESM-ified

pubsub-simple.js

let topics = {}, subUid = -1;
// https://gist.github.com/fatihacet/1290216
export default {
    subscribe(topic, func) {
        if (!topics[topic]) {
            topics[topic] = [];
        }
        let token = (++subUid).toString();
        topics[topic].push({
            token,
            func
        });
        return token;
    },

    publish(topic, args) {
        if (!topics[topic]) {
            return false;
        }
        setTimeout(() => {
            let subscribers = topics[topic];
            let len = subscribers ? subscribers.length : 0;

            while (len--) {
                subscribers[len].func(topic, args);
            }
        }, 0);
        return true;
    },

    unsubscribe(token) {
        for (let m in topics) {
            if (topics[m]) {
                for (let i = 0, j = topics[m].length; i < j; i++) {
                    if (topics[m][i].token === token) {
                        topics[m].splice(i, 1);
                        return token;
                    }
                }
            }
        }
        return false;
    }
};

I asked ChatGPT to write the same kind of library and it gave me this.

const topics = {};

export function subscribe(topic, callback) {
  if (!topics[topic]) {
    topics[topic] = [];
  }
  topics[topic].push(callback);
}

export function publish(topic, data) {
  if (!topics[topic]) {
    return;
  }
  topics[topic].forEach(callback => {
    callback(data);
  });
}

export function unsubscribe(topic, callback) {
  if (!topics[topic]) {
    return;
  }
  const index = topics[topic].indexOf(callback);
  if (index !== -1) {
    topics[topic].splice(index, 1);
  }
}