Parallelism means using multiple hardware resources on some task. Concurrency means multiple tasks may make progress in the same time period - perhaps via interleaving.
What I'm saying is that JS is terrible at allowing multiple tasks to make progress when one of those tasks is computation. Try taking some numerical algorithm and allowing interleaving user events with it - it's super nasty.
I hope I'm wrong because I've struggled with this. How would you perform a JS computation concurrently with handling user events?
I'm not a JS expert, but I do a lot of work with heavy numerical computing that occasionally has to run inside a responsive UI without threading (it's no fun). The solution is "simple", if annoying:
1) you find a small-enough granule of work inside each compute kernel so that one iteration/update/whatever of it is under the target frame duration (for these kinds of UIs 30 FPS is plenty and 15 FPS is tolerable, we're talking business visualization apps and so on)
2) you organize your computation as a resume/yield loop [0]; you run a few iterations of a function like "percent_finished = make_progress(state_of_computation)" then schedule yourself to do another round of progress on the next frame, and return/yield so that the event loop / UI can flush any pending I/O events
Needless to say, this gives up a lot of compute efficiency in return for making it relatively easy to maintain responsiveness. And it doesn't work on problems where you can't find good "yield" points in the computation. But a lot of numerical computing problems have natural/reasonable yield points, e.g. after each data point is processed, or after every 100 elements of a large vector are processed.
[0] A great example of this in a widely used library is the streaming compression interface in zlib, which lets you compress or decompress a nearly-arbitrarily-small granule on each invocation.
What I'm saying is that JS is terrible at allowing multiple tasks to make progress when one of those tasks is computation. Try taking some numerical algorithm and allowing interleaving user events with it - it's super nasty.
I hope I'm wrong because I've struggled with this. How would you perform a JS computation concurrently with handling user events?