The documentation here is insufficient:

http://api.jquery.com/toggleclass/#toggleClass-state

Also, the current behavior does not seem consistent from what one might expect by looking at the other signatures:

https://jsfiddle.net/jdoedx2v/4/

It seems the behavior of toggleClass() and toggleClass(true) are the same. I'd expect toggleClass(true) to always restore the original classes from the first call the same way as toggleClass(false) would always remove all original classes. But maybe I'm missing something because there's no proper documentation and I couldn't understand the real intention here...