Simple JS event PubSub implementation

events.js

/**
 Events, v2.0.0
 Created by Anton Horodchuk for the ABillS Framework, http://abills.net.ua/
 MIT License
 
 Simple JavaScript Event PubSub implementation

 One can install listener for event with 'on'

   events.on('eventhappened', function(data){ console.log(data) });
   events.emit('eventhappened', 'Hello, async world') // 'Hello, async world'

 Sometimes you need to remove listeners in long time running application.
 There are three separate interfaces for it.
   1. Use returned object
     var eventHandler = events.on('eventhappened', function(data){ console.log(data) });
     // Do something here
     eventHandler.remove();

   2. Use named function and remove it explicitly
   events.on('eventhappened', doSomeEventBasedAction);
   // Later in your code
   events.off('eventhappened', doSomeEventBasedAction);

   3. Remove all listeners for event (Use with care)
   events.off('eventhappened');

 Also you can set one time callback (will remove itself after call)
   events.once('specificevent', doSomeEventBasedAction);
 
 */

'use strict';

var Events = (function () {
  var debug  = 0;
  var topics = {};
  
  /**
   * Set listener for event
   * @param topic
   * @param listener
   * @returns {{remove: remove}}
   */
  function on(topic, listener){
    if (debug > 0) console.log('[ Events ] listen to :', topic);
  
    topics[topic] = topics[topic] || [];
    var index     = topics[topic].push(listener) - 1;
  
    // Return interface to remove self
    return {
      remove: function () {
        delete topics[topic][index];
      }
    }
  }
  
  /**
   * Set listener that will be removed after call
   * @param topic
   * @param listener
   */
  function once(topic, listener){
    if (debug > 0) console.log('[ Events ] once to :', topic);
    
    on(topic, function (data) {
      listener(data);
      off(topic, listener);
    });
  }
  
  /**
   * Remove given listener for event or all if no listener specified
   * @param topic
   * @param listener
   */
  function off(topic, listener){
    if (topics[topic]) {
      if (typeof listener === 'function') {
        for (var i = 0; i < topics[topic].length; i++) {
          if (topics[topic][i] === listener) {
            topics[topic].splice(i, 1);
            break;
          }
        }
      }
      else {
        // Clear all listeners
        topics[topic] = [];
      }
    }
  }
  
  /**
   * Run all subscribed listeners
   * @param topic
   * @param data
   */
  function emit(topic, data){
    if (debug != 0) console.log('[ Events ] emmitted :', topic);
  
    if (topics[topic]) {
      topics[topic].forEach(function (fn) {
        fn(data);
      });
    }
  }
  
  return {
    on : on,
    once : once,
    off  : off,
    emit : emit,
    debug: debug
  }
}());