Publisher in Pub/Sub should be Synchronous or Asynchronous?

My view on mechanism:

• Pattern wise publisher should just publish an event and should not care about listeners / subscribers for that topic / event.
• In case of JavaScript, lot of frameworks for Pub / Sub publishes event in Sync manner
• for example : framework just maintains map of event to function.
• on particular event occurrence, it iterates over subscribers and calls function one by one - > function_1.apply(context, args) and then function_2.apply(context, args)
• This makes it Synchronous as, unless and until function_1 finishes, function_2 is unaware that particular event happened.

What is an ideal way of implementing Pub / Sub in JavaScript ?