Learn to write by tracing letters (Processing.js)
Live demo http://andrewringler.github.io/letter-trace/
Letter Trace was written with Processing 2 Javascript Mode. Which allowed you to export Processing Sketches to Javascript. The easiest way to continue development is to download Processing 2, enable Javascript Mode and load Letters/Letters.pde.
Unfortunately there is no Javascript Mode for Processing 3. If you are a web developer you could just start using the files in Letters/web-export, they are all valid js files. Just launch Letters/web-export/index.html from Firefox and you will see it running locally.