Heartbeat (MIT) is a MIDI/Audio sequencer written in javascript. Heartbeat has no GUI. It is intended to be used as an engine behind your application.
Heartbeat is set up very flexible so you can make any kind of application on top of it; a game, an online DAW, artistic sites, music science experiments and so on.
So far heartbeat has been used in 2 MusicFirst projects: Groovy and Morton Subotnick’s Music Academy.
Check the list of key features.