After PHP, one of the most common tools I use for development is jQuery. JQuery is a library add-on to enhance javascript while also making it a ton easier to use. From the developers themselves:
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.
Web pages with HTML alone are very static displays. You load it once and you are done. Javascript is the most common language for taking a static web page and making it dynamic. For example, compare the GUI like interface of a modern website like Gmail to an old-fashioned MySpace page from the 90s.
JQuery is a great way to get into the javascript world. My company is currently upgrading our jQuery from an obsolete 1.9 to the newest 3.3 version so I thought it was time to brush up on my skills. While I can jump right in, I wanted to start from the bare minimum for my readers. I am going to assume that you know the basics of HTML at this point, but that is all. First start here to get a brief overview of jQuery. Now these articles get pretty heavy pretty fast, but it is a good place to start. The next thing you'll need is a good reference for selectors. I actually prefer the w3schools reference to the selectors, which I still refer to in my day job.
Now that I've given you the basic references, time to put your knowledge into practice. I firmly believe that one can read all you want about programming, but until you actually start writing some code, your knowledge won't stick with you. I've mentioned in previous posts about how I like self-grading assessments(like Advent of Code) as a fun way to learn code, and the jquery sit provides it's own at http://try.jquery.com/.
It has a nice point-based gamification as it slowly ramps up the difficulty of the tasks and reward points for completing tasks. I've been working through it myself as a refresher course. I recommend working through the basics on this site, before I go into more advanced subjects
I'm going to end this post here, but I hope to get into more depth with future posts as well as introduce jQuery UI, which is an additional addon to jQuery that adds a ton of graphical widgets that can be used on your web page. Please add any questions or requests in the comments below and happy javascripting.
All images from jquery.com.