16. create an Event Emitter

There is Event Emitter in Node.js, Facebook once had its own implementation but now it is archived.

You are asked to create an Event Emitter Class

const emitter = new Emitter()

It should support event subscribing

const sub1  = emitter.subscribe('event1', callback1)
const sub2 = emitter.subscribe('event2', callback2)

// same callback could subscribe 
// on same event multiple times
const sub3 = emitter.subscribe('event1', callback1)

emit(eventName, ...args) is used to trigger the callbacks, with args relayed

emitter.emit('event1', 1, 2);
// callback1 will be called twice

Subscription returned by subscribe() has a release() method that could be used to unsubscribe

sub1.release()
sub3.release()
// now even if we emit 'event1' again, 
// callback1 is not called anymore