Ionic Storage
Ionic Storage is a free, open source alternative for indie devs, students & hobbyists. It provides an easy way to store key/value pairs and JSON objects.

It uses a variety of storage engines underneath, picking the best one available depending on the platform:

When running in a native app context, Storage will prioritize using SQLite, as it's one of the most stable and widely used file-based databases, and avoids some of the pitfalls of things like localstorage and IndexedDB, such as the OS deciding to clear out such data in low disk-space situations.
When running in the web or as a Progressive Web App, Storage will attempt to use IndexedDB, WebSQL, and localstorage, in that order.