There are tools available for sketching drawings online, but no tools that I know of for sketching music. One of the problems is that it's much easier to make the former, of course. But perhaps the imagination is also lacking.
It would be nice to use the keyboard to give chords a bit like playing four note chords on a guitar. The range of patterns that you use on a guitar for jazz chords is not all that diverse, but sticking them all together makes good music. Getting the inversions right gives music a lot of its quality.
Unfortunately, a lot of the quality of music is from having proper acoustics. You can get some good sounds with advanced music software now, but the pathetic whines of a JavaScript horn are probably not going to rival even low end MIDI synthesis from the 1990s.
Still, if you remember the SID chip from the 1980s, perhaps you can imagine something like it being programmed in JavaScript too. A modern JSID, so to speak.