Возможно, объект?
const titles = {
'name': ['name', 'idea', 'ide', 'ide navn', 'title', 'idea name'],
'description': ['beskrivelse', 'problemet', 'description', 'the problem', 'ide beskrivelse'],
'owner' : ['ejer', 'owner', 'opfinder', 'ide person', 'idea person', 'person'],
'duedate' : ['duedate', 'deadline', 'tidsfrist', 'sidste dato', 'dato', 'due date'],
'imageUrl' : ['billede', 'billeder', 'image', 'images', 'attachment']
}
const getKey = (obj,val) => Object.keys(obj).find(key => obj[key].indexOf(val) !=-1 );
function findHeader(object) {
var title = object.toString().trim().toLowerCase();
return getKey(titles,title) || 'Unassigned'
}
console.log(
findHeader("Owner"),
findHeader("Bla")
)