Integrate External, Read-Only Data Source Into Reactive System

// Hypothetical external data
  let externalData = new Map()
  EventEmitter.emit("dataChanged", /*no event info*/)

// My problem...
  function externMemo (externData) {
    EventEmitter.on("dataChanged", ()=>/*trigger getter refetch & dom update*/ )
    let getter = ()=>{ 
      // ...
      return externData 
    }
  
    return getter
  }

// Hypothetical Component
  let component = () => {
    let reactiveData = externMemo(externalData)
    return (
     <div>
       {reactiveData().values() /*returns current externalData values*/}
     </div>
    )
  }