This is a candidate for the reference javascript implementation of the Porter2 stemming algorithm, (rev. July 2005).
There is a second project which covers the original Porter Stemmer from 1980, you can find that at the following url: https://github.com/kristopolous/Porter-Stemmer.
The reference has also been included in the spec directory.
Include the js file, then run stemmer(<word to stem>) to return the stemmed word.
You can get a trace as to what is going on, corresponding to the steps outlined in the papers, by provding a second argument of "true" and using one of the browser debug consoles.
For instance, stemmer("hopefully", true) will output 1c /^(.*[aeiouy].*)y$/ hopefulli, telling us that rule 1c was matched
by that regular expression and as a result, we got hopefulli.
As of Oct 22, 2012, PorterStemmer2 is 98.54% compliant.