You could use a function like this:
function getById (path, ids) {
return firestore.getAll(
[].concat(ids).map(id => firestore.doc(`${path}/${id}`))
)
}
It can be called with a single ID:
getById('collection', 'some_id')
or an array of IDs:
getById('collection', ['some_id', 'some_other_id'])