That sounds completely backward. It would probably just be easier to port the DSP code over to C++.
You could then compile that DSP code with Emscripten to convert it to Js, for use in the website. This gives you the added benefit of having JUCE’s great audio/dsp classes. Emscripten does support WebAudio and I know a few people on the forums have played with that.
Doing it the other way around just seems perverted.
If you really want to dive into that rabbit hole, you would probably want to start here:
I shudder at the thought of it being used during the audio thread, but it might work depending on how complex the filter is.
But seriously, just port it to C++. You have all the tools you’d ever need to do that within JUCE. I just looked at the AudioLib.js library, and JUCE has a replacement for everything in that library and more, plus you’d be at native speed.