Persist events that not yet have a handler

[AndreasJilvero]

There's a risk that a event is emitted before its' handler has been registered. You might say that such code is bad design, but I'd like to be lazy ;) Also, given that code splitting is being more and more adopted, I think this scenario is very real.

Would it be possible to persist events that don't have a handler (yet)? Whenever .on(...) is run, mitt would then check the persisted events in order to find unprocessed ones, and then handle those events immediately.

I think the footprint for this change can be quite small, but I can't speak for performance. Maybe we can opt-in to the behaviour.

[phoenix-ru]

I think you can emulate it with a simple queue and a catch-all, something like

const queue = new Map<string, unknown>()
emitter.on('*', (type: string, ev: unknown) => {
  // Disable processing when event is already registered
  if (emitter.all.has(type)) return

  if (!queue.has(type)) queue.set(type, [])
  queue.get(type).push(ev)
})

And then, when you are finally ready, add the necessary handler and flush the queue:

function addHandler(type: string, handler: (v: unknown) => void) {
  emitter.on(type, handler)
  ;(queue.get(type) || []).forEach(v => emitter.emit(type, v))
  
  // Free the resources
  queue.delete(type)
}