Subject.js

let Subject = function(subscribersStateChangeNotificationCallback) {
	let self = this;
	
	let handlers = {};
	
	Object.defineProperty(self, "subscribersFound", {
		get() {
			let found = false;
			
			for(const prop in handlers) {
				if(handlers.hasOwnProperty(prop)) {
					found = true;
					break;
				}
			}
			
			return found;
		}
	});
	
	Object.defineProperty(self, "subscribersCount", {
		get() {
			let count = 0;
			
			for(const prop in handlers) {
				if(handlers.hasOwnProperty(prop)) {
					count++;
				}
			}
			
			return count;
		}
	});
	
	let unsubscribeNotificationCallback = (handlerId) => {
		if(handlerId && handlerId !== '' && handlers.hasOwnProperty(handlerId)) {
			delete handlers[handlerId];
			
			if(subscribersStateChangeNotificationCallback && !self.subscribersFound) {
				subscribersStateChangeNotificationCallback(false);
			}
		}
	};
	
	self.subscribe = (handler) => {
		let handlerId = createGuid();
		handlers[handlerId] = handler;
		
		if(subscribersStateChangeNotificationCallback && self.subscribersCount === 1) {
			subscribersStateChangeNotificationCallback(true);
		}
		
		return new Subscription(handlerId, unsubscribeNotificationCallback);
	};
	
	self.next = (data) => {
		for(const handlerId in handlers) {
			handlers[handlerId](data);
		}
	};
	
	return self;
};

let createGuid = function() {  
   return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c) {  
      var r = Math.random()*16|0, v = c === 'x' ? r : (r&0x3|0x8);  
      return v.toString(16);  
   });  
};