My take on the Signal event concept

makeSignal.js

export const makeSignal = () => {
  const subscriptions = new Map()

  const signal = {
    get size () {
      return subscriptions.size
    },
    subscribe (listenerFn) {
      if (subscriptions.has(listenerFn) === false) {
        if (typeof listenerFn === 'function') {
          const pkg = {
            isOnce: false,
            listenerFn
          }
          subscriptions.set(listenerFn, pkg)
        } else {
          console.error('ERROR: Subscribe requires a function as a paramater.')
        }
      }
      return () => {
        if (subscriptions.has(listenerFn)) {
          subscriptions.delete(listenerFn)
        }
      }
    },
    subscribeOnce (listenerFn) {
      if (subscriptions.has(listenerFn) === false) {
        if (typeof listenerFn === 'function') {
          const pkg = {
            isOnce: true,
            listenerFn
          }
          subscriptions.set(listenerFn, pkg)
        } else {
          console.error('ERROR: SubscribeOnce requires a function as a paramater.')
        }
      }
      return () => {
        if (subscriptions.has(listenerFn)) {
          subscriptions.delete(listenerFn)
        }
      }
    },
    emit (...props) {
      for (const subscriber of subscriptions.values()) {
        const { isOnce, listenerFn } = subscriber
        listenerFn(...props)
        if (isOnce) {
          subscriptions.delete(listenerFn)
        }
      }
    },
    clear () {
      subscriptions.clear()
    }
  }

  return signal
}