If you asked this five years ago, jQuery would have been the hands-down answer because functionality in general was implemented so differently/poorly/not-at-all across the various browsers (and one really really bad browser in particular).
You will never go wrong with an understanding of vanilla JS. As for jQuery, you will not need jQuery if you get into React. (okay so... I have a React project that includes jQuery because of Semantic UI... but pretend what I said before is true).
Also, what @lichtjaeger said... look at the new 'fetch' functionality.