I think JS should be higher on the priority list, considering the demand to effort ratio (I'm thinking it's probably easier to implement for JS than Java). I would guess that it would also be easier to host it and load JS on the fly to debug online. Just my 2c, will try the Java one anyway to test it out.
I totally agree on the demand, but on the effort side the dynamic nature of JS makes it really tricky to do right. Thanks for your comment & let me know how it goes!
Take a look at GraalJS/Truffle instrumentation. It lets you easily run JS and even NodeJS on the JVM with per statement instrumentation handled at the engine layer. So it should make it very easy to add the code you need and the compatibility with modern js is great.
The critical problem is not the instrumentation, but the way how BugJail works: it doesn't dumb the raw events to disk, instead it reconstructs relational model of the program execution. This requires resolving raw low level events against schema of types/methods/fields/etc. With JS the schema is dynamic, so now the already hard 'reverse-engineering' has moving target. Not impossible, just hard.
And thanks for the pointer to GraalJS/Truffle, I'll have look and perhaps that provides great way for doing the instrumentation side.
