Google Lovefield is a JS relational database layer on top of IndexedDB: https://github.com/google/lovefield
In their FAQ, it is said to be (or have been) in use by gmail.
It seems to no longer be developed and hardly maintained, and while the docs are not bad, I couldn't find a lot of information / tutorials for it.
I also didn't find information on whether it has been abandoned, or if it is continued or was transformed into another library.
If anyone has concrete information about the state of this library or derived products then I'll be happy to know.
According to comments from GitHub, it is in "long-term maintenance mode".
This means basically abandoned, but good enough for production for now. https://github.com/google/lovefield/issues/266#issuecomment-678883485
https://github.com/google/lovefield/issues/270#issuecomment-708864795