If you don't care about latency, and in this "DSP sandbox" application you probably don't, JavaScript is fine for real-time DSP. JS under V8 and similar engines can be quite fast.
You aren't going to digitize 100 MHz of RF spectrum and build an SDR in JavaScript, but for audio rendering work it'd be fine. Cool hack.
I'm assuming that a full-fledged DAW cares a lot about latency. Live performance becomes difficult when latency creeps much above 5ms, for example. A lot of things that you normally don't think about (like when to allocate a buffer of memory) become critically important with low latency requirements.
That isn't an issue for the demoed app, but it is for the hypothetical longer-term goal under discussion.
You aren't going to digitize 100 MHz of RF spectrum and build an SDR in JavaScript, but for audio rendering work it'd be fine. Cool hack.