indexOf returns -1 on not found value, not 0, thus it has to be explicitely compared to it in determining whether to apply a prefix.
It seems this method isn't used anywhere but if it stays it should probably be fixed. :)
P.S. Sorry for closing the previous pull request, I suck at using GitHub, apparently.
Sorry I hadn’t replied yet, looks like I missed this pull request. Why did you close it? I can’t see any commits in it, are there any?
I stupidly commited my change to the main gh-pages branch which makes it hard to keep track of the upstream repo so I had to revert it (contributing to jQuery has tought me the proper github pull request work flow, thankfully, though).
If I understand it correctly, there's a mistake in line 235: you should prepend self.prefix if self.properties.indexOf(property) is equal to -1, not if it's truthy. However, I don't see anything actually using this method so maybe it can be removed as well. But maybe you just wanted to expose it as part of API.
I guess there's no need for a new pull request - it's a cosmetic change so you can do it yourself if my remarks were right. :)