Is such a file system possible?

Currently the file systems on web are MEMFS and IDBFS and WORKERFS.

MEMFS: all files are load into memory. If the files are large, a lot of memory will be comsumed.
IDBFS: for sync between MEMFS and IndexedDB. It is just the mirror of the MEMFS
WORKERFS: Looks like the fs is managed in a worker. Not able for a multi-threaded application.

I was wondering if such a FS be possible:
1. for all files opened: load into memory
2. for all files not opened: Keep in the IDBFS and not load into memory

If it is possible the memory usage will be much less than MEMFS and we can still get the files persistent storaged.