Emitter

emitter.js

const createEmitter = () => {
  const handlersByEvent = new Map()

  const on = (event, handler) => {
    let handlers = handlersByEvent.get(event)
    if (!handlers) {
      handlers = []
      handlersByEvent.set(event, handlers)
    }
    handlers.push(handler)
    return () => {
      const index = handlers.findIndex(_handler => _handler === handler)
      if (index < 0) return
      handlers.splice(index, 1);
    }
  }

  const trigger = (event, ...args) => {
    let handlers = handlersByEvent.get(event)
    return (handlers || []).map(handler => handler(...args))
  }

  return {
    on,
    trigger,
  }
}