I experimented with various approaches to client-side data handling and ended up realizing that I do not want a client-side database at all. I'd much rather hold data in language-native data structures (in my case, Clojure/ClojureScript). These are very powerful and there is zero impedance mismatch: after all, they are a part of the language.

Yes, you do need queries, but there are not many and they might as well be coded as functions. You can do all the indexing you want, if you need to, but JavaScript in browsers is so incredibly fast these days, that even a linear search works well for most cases.

Unless you are handling gigabytes of data client-side, of course. But that wouldn't make much sense.

Data modification needs to go through a server API anyway, because the client environment can't be trusted, so two-way sync would be useless. I want the server to perform data model operations while verifying everything, I do not want data sync.