JS (for the browser anyway) is difficult in that there's basically three generations, and any docs you read are may be from any of them:

1. Per browser DOM APIs

2. jQuery

3. Modern DOM APIs. document.querySelector(), nodelist.forEach(), element.classList() etc. You'd use CSS transitions for animations you used to do in jQuery.

Personal style also comes into it. Some people use function variables, others functions declarations. Some people put script tags everywhere, others use one of two modules systems. There's no one right way to do things. But there's a lot of wrong ones.

First things first: understand the basics, like async and closures. They haven't changed.

And get the yellow book with the bird.