Publish-subscribe implementation with special features.

PubSub.js

var PubSub = (function PubSub() {

    var subscriptions = {};
    var publications = {};
    var index = 0;

    function sub(event, callback) {

        // If already subbed, return the id
        var subId = getSubId(callback);
        if (subId) return subId;

        if (typeof callback !== 'function') throw "Callback not a function";

        if (subscriptions[event]) {
            subscriptions[event][index] = callback;
        } else {
            subscriptions[event] = {};
            subscriptions[event][index] = callback;
        }
        index++; // Might break after 9007199254740992 iterations
        return index - 1;
    }

    // Send data to all subscribers of an event
    function pub(event, data) {
        // If no data is passed, make it a null to indicate that it was called, nonetheless
        publications[event] = data || null;
        for (var key in subscriptions[event]) {
            subscriptions[event][key](data);
        }
    }

    // Subscribe to the event and get last publication (if available)
    function then(event, callback) {
        var subId = sub(event, callback);
        // If it was previously called, but no data was passed
        // the value will be null
        if (typeof publications[event] !== 'undefined') callback(publications[event]);
        return subId;
    }

    // Unsubscribe a function from an event
    function unsub(subId) {
        if (typeof subId === 'function') subId = getSubId(subId);
        for (var eventKey in subscriptions) {
            for (var subKey in subscriptions[eventKey]) {
                if (subKey === subId) {
                    delete subscriptions[eventKey][subKey];
                    return true;
                }
            }
        }
        return false;
    }

    function unsubAll(subArray) {
        subArray.forEach(unsub);
        console.log(subscriptions);
    }

    // Get last publication (or next if no last exists) only
    function once(event, func) {
        var wrapper = function(data) {
            func(data);
            unsub(wrapper);
        };
        return then(event, wrapper);
    }

    // Get next publication only
    function next(event, func) {
        var wrapper = function(data) {
            func(data);
            unsub(wrapper);
        };
        return sub(event, wrapper);
    }

    // Look through the list and get the function
    function getSubId(func) {
        for (var eventKey in subscriptions) {
            for (var subKey in subscriptions[eventKey]) {
                if (subscriptions[eventKey][subKey] == func) {
                    return subKey;
                }
            }
        }
        return null;
    }

    return {
        sub: sub,
        once: once,
        then: then,
        next: next,
        pub: pub,
        unsub: unsub,
        getSubId: getSubId,
        unsubAll: unsubAll
    };

})();