Although the Processing.js (Pjs) library only got a builtin Java-to-JS transpiler, there’s a language similar to Python & Ruby called CoffeeScript which also transpiles to JS: https://CoffeeScript.org/
On the old Processing 2.x.x, we had an installable flavor called CoffeeScript Mode:
Instead of Java syntax, we’d write our sketches using CS syntax, which would get transpiled to JS’ for deploying on the web w/ the Pjs library. 
We can still grab Processing v2.2.1 from here: 
https://Processing.org/download/
Just be extra careful to assign a diff. Sketchbook folder path for the old P2; so P2 & P3 libraries won’t crash each other! 