which one? and why?

get_set.js

// #1 - getter & setter
service.getPriceUpdatedAt = () => store.priceUpdatedAt

service.setPriceUpdatedAt = (tmstmp) => {
  store.priceUpdatedAt = parseInt(tmstmp)
}

// #2 - 2in1
service.priceUpdatedAt = (tmstmp) => {
  if (!tmstmp) return store.priceUpdatedAt
  else store.priceUpdatedAt = parseInt(tmstmp)
}

I like the second one, however i could embrace anything in use if there is an existing codebase

I prefer the first, is auto explained and avoid mistake calls

and an third way is

service.priceUpdatedAt = {
  get: () => store.priceUpdatedAt,
  set: (tmstmp) => {
    store.priceUpdatedAt = parseInt(tmstmp)
  }
}