Fixed it! Thanks!

> ...wondered if there was a simple way to calculate symbolic derivatives on a web page without relying on a server. I tried a couple of JavaScript libraries that should do the trick, but they didn't work as expected; to make matters worse, they were huge and seemed to be unmaintained.

from: https://github.com/hausen/math.diff.js

I realise I had to go digging for that, however I was looking at a similar problem recently too.

http://www.numericjs.com/ is a similar project, but perhaps that's what the author was referring too with large and out of date.

There is also http://mathjs.org/, which doesn't support symbolic maths, but has a parser and some work already done. It could be worth adding to that project.

Any change you are aware of the source code for the project you linked? I couldn't find any reference to it..

nice!