In-memory JavaScript databases

NoSQL

PouchDB
https://pouchdb.com/

RxDB
https://github.com/pubkey/rxdb (based on PouchDB)

TaffyDB
http://taffydb.com/

LokiJS
http://lokijs.org

NeDB
https://github.com/louischatriot/nedb

RxDB
Real-time
https://github.com/pubkey/rxdb

SQL

sql.js

https://github.com/kripken/sql.js/

AlaSQL

http://alasql.org/

WebSQL

Dead!

https://www.w3.org/TR/webdatabase/