You can check whether element.className
matches /\bthatClass\b/
.\b
matches a word break.
Or, you can use jQuery’s own implementation:
var className = " " + selector + " "; if ( (" " + element.className + " ").replace(/[\n\t]/g, " ").indexOf(" thatClass ") > -1 )
To answer your more general question, you can look at jQuery’s source code on github or at the source for hasClass
specifically in this source viewer.