fyprocessing:

The main goal of Processing.js is to execute Processing files in HTML5, but not necessarily to write native HTML5. It supports a mixed syntax of Processing and JavaScript, where the JavaScript is not really meant to be consumed by the end-user. Processing.js is a port of Processing to JS, using regex to convert Java into JS. It is a good tool for those that want to run simple sketches on the web, however, it is quite opaque. It can be difficult for someone to understand how it works, how to fix things when it doesn’t work, or how to modify or extend the library. As Processing.js says on their website, “it’s not magic, but almost.”

In contrast, with p5 we are reimagining Processing’s original goals in native JavaScript, in a way that is intended to be transparent and intuitive. It is easy to translate a sketch from Processing to p5 and the process of doing so begins to teach people the basics of JS. From there, they are able to begin writing native JS using a syntax that feels familiar but appropriate for the context. We are closely studying the decisions Processing has made, but also always questioning to see if there are design decisions that would make the library cleaner, stronger, and more intuitive…  

(Nice explanation.)