I don’t know whether JMESS (http://jsmess.textfiles.com/) has any similar component (I should check in order not to re-invent the wheel) but floppyJS mainly aims inspecting/editing floppy images.
Maybe in later stages a disk image API to combine with any JS emulator may be developed, who knows…
Currently the code is rather messy and only supports FAT12 images. If it raises any attention and if anybody is interested, a more detailed and systematical coding scheme can be introduced with some documentation.
You can reach it on github