I'd like to start with something that's more DB oriented like SQLite both locally and in the browser for example but I understand a documented-oriented approach might make more sense.
RealmDB is sort of the goto. They were acquired by mongo which could be a negative signal but the team is solid and I haven't seen it result it much degredation.
EDIT: actually... it looks like mongo may have just announced the EOL for server-side component a couple weeks ago... bad timing!