What is the “hasClass” function with plain JavaScript?

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.

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