Processing.js is more than just a Processing parser re-written in JavaScript. It is designed in a way that connects the Processing language (also known as P5) with web technologies such as JavaScript, the HTML5 canvas element, JQuery, and various web services. Furthermore, Processing.js is built in such a way as to allow easy integration of new technologies as they emerge. It is designed to be fast and to take advantage of recent JavaScript developments to ensure that the platform is responsive.

While syntactically JavaScript and Java are fairly similar, there are some fundamental differences that has made this conversion challenging. The first is that we wanted to do this conversion dynamically in real time. The code produced by the converter needed to be fully object oriented and we had to provide support to all native Java functions and objects (such as Strings) that are supported by Processing. We also had to take into account the differences between working with web resources vs local resources. Furthermore we had to consider how we would handle some fundamental differences between Java and JavaScript such as typed vs. typeless variables, function overloading and variable name overloading. The original code for Processing.js used regular expressions to convert Java into JavaScript when it was encountered. It did this by scanning for hints of Java code within the entire sketch and then replaced the Java code with its JavaScript.