By definition, Processing.js (or Javascript mode) uses Javascript, not Java, and most libraries are built against Java and thus will not work with Processing.js. Only the core Processing functions were converted to Javascript, and a few commonly used Java classes, such as ArrayList. My guess is that ControlP5 doesn't work with Javascript. I might be wrong... but that would mean that you would need to find a different library that works with Javascript.

Hi, at this point there is no javascript compatible version of controlP5 for processingjs which I believe is the case for most libraries at this point since you need to rewrite your library in javascript in order to work with processingjs. In case you need a slider only, have you considered to make use of the HScrollbar class used in the Scrollbar example that comes with processing?

Thanks andreas.  That HScrollbar should work fine for now.