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

pubsub-simple.js

var pubsub = {};
(function(q) {
    var topics = {}, subUid = -1;
    q.subscribe = function(topic, func) {
        if (!topics[topic]) {
            topics[topic] = [];
        }
        var token = (++subUid).toString();
        topics[topic].push({
            token: token,
            func: func
        });
        return token;
    };

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

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

    };

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

I don't understand why notifying the subscribers is happening within a setTimeout. Is it because you want to return the publisher before the subscribers are notified / handled which can be potentially expensive?

@rvenugopal - the code inside setTimeout will be executed asynchronously, so normal code flow(where it is publishing) won't halt.

@brajendraSwain While publishing, I'd prefer to have a loop, and for each subscriber, do the magic with setTimeout, to not block the other subscribers if one is doing heavy processing

var subscribers = topics[topic],
    len = subscribers ? subscribers.length : 0;
while (len--) {
    var notifier = (function(s, t, a) {
        return function() { s(t, a); };
    })(subscribers[len].func, topic, args);

    setTimeout(notifier, 0);
}

Starting from this implementation, one can easily manage a silent reverse pub/sub to add bidirectionality :
https://github.com/frufin/js-bidirectional-pubsub

@mocheng (nearly a year after your question...) The for...in loop allows an individual subscription to be cancelled (via the token returned upon subscribing). Removing the loop would destroy the event entirely, rather than a single subscription. A way around that might be to specify some ID token in the event like click.token. I'm not sure what arguments exist for which is better/worse.

Additionally, in your example, if (topics[m]) would fail because m is not defined without the loop. I'm guessing you meant something like topics[event] where event would be passed through the unsubscribe() params. For example:

// object definition
unsubscribe: function(event) {
    // ...
    if (topics[event]) {
    // ...
}
// code using pubsub
pubsub.unsubscribe('myAwesomeEvent');

With all of that said, you've probably already come to this conclusion after a year or don't care and have totally forgotten about this code. Either way, it was a good exercise for me ^_^

some questions here!
If my thought was wrong, please tell me.
I am new to javascript!

https://gist.github.com/fatihacet/1290216#file-pubsub-simple-js-L17-L27

1. I think it could be more simpler.

var subscribers = topics[topic];
if (!subscribers) {
    return false;
}

setTimeout(function(){
    for(var i=0, len = subscribers.length; i<len; i++) {
        subscribers[len].func(subscribers, args);
    }
}, 0);

https://gist.github.com/fatihacet/1290216#file-pubsub-simple-js-L34
2. and this conditional statement looks like unnecessary

https://gist.github.com/fatihacet/1290216#file-pubsub-simple-js-L22
3. also I can't understand why he used conditional statement here.
Is it unnecessary that we checked the conditional statement for undefined from the top?

ps. thank you for nice implementation. It was really helpful.

While above is good, and it's crazily 7 years old. I try below method. Instead of using an array, I use an object for each subscriptions as well. Simpler and smaller Also note the unsubscribe closure return in publish, kinda helps.

PubSub.js

class PubSub {
  constructor () {
    this.subIds = 0;
    this.subscriptions = {}
  }
  
  subscribe (topic, fn) {
    if(!this.subscriptions[topic]) this.subscriptions[topic] = {}
    const token = ++this.subIds;
    // Validate topic name and function constructor
    this.subscriptions[topic][token] = fn;
    
    return () => this.unsubscribe(topic, token)
  }
  
  publish (topic, ...args) {
    const subs = this.subscriptions[topic];
    if(!subs) return false
    Object.values(subs).forEach(sub => sub(...args))
  }
  
  unsubscribe (topic, token) {
    if(!token) delete this.subscriptions[topic]; // Delete all subscriptions for the topic
    this.subscriptions[topic] && (delete this.subscriptions[topic][token]); // Delete specific subscription
  }
}

export default new PubSub();

Example

const unsub1 = PubSub.subscribe('spacex', data => console.log('Falcon was launched', data));
const unsub2 = PubSub.subscribe('spacex', data => console.log('Falcon Heavy was launched', data));
PubSub.publish('spacex', 'some data slash params')

// Unsubscribe single subscription
unsub1(); // Unsubscribes Falcon
unsub2(); // Unsubscribes Falcon Heavy

// Unsubscribe ALL subscriptions for a topic
PubSub.unsubscribe('spacex')

This isn't publish subscribe pattern, this is the simplest observer pattern.

How can I use this as a subscriber

@scazzy thank you it's precisely what i needed