When JavaScript feels difficult, this is a good lecture to listen to. I use JavaScript often and it doesn't interfere much in my work, but when I actually try to understand it, there are many difficult parts. They say they use ES6+ because it's the latest, but it ultimately operates on top of the ES5 mechanism, so if you try to understand it with only ES6+, you'll only understand it superficially. This lecture seems to explain those parts well.