javascript - immutable js - create or update a list nested inside a map -

i have modify system picking assets allow user pick multiple assets. add 1 asset id production id map this:

.setin(['newrequest', 'productionassets', action.productionid], action.assetid) 

how can modify create new list if first, or append if not first?

i ended doing 2 rounds. there maybe more succinct version, works

if (s.hasin(['newrequest', 'productionassets', action.productionid])) {         return s           .updatein(['newrequest', 'productionassets', action.productionid], (prodassets) => prodassets.push(action.assetid))                   } else {         return s           .setin(['newrequest', 'productionassets', action.productionid], fromjs([action.assetid]))                    }