jQuery
The predominant JS library, learn jQuery to help do just about anything on a website or HTML-based application. Learn how to animate elements, manipulate the HTML DOM, work with data, make AJAX calls, and much, much more.
What is jQuery?
jQuery is a fast, small, and feature-rich JavaScript library. It makes things like HTML document traversal and manipulation, event handling, animation, and Ajax much simpler with an easy-to-use API that works across a multitude of browsers. With a combination of versatility and extensibility, jQuery has changed the way that millions of people write JavaScript.