In one of these lives, you will be an ES5 programmer, bound by the traditions of yore, you sidestep a handful of edge cases with quiet dignity; you know there may be a better way, but you keep your head down, perform some perfunctory syntax rituals, and get the job done.
In your other life, you are an ES6 programmer. You have no time for verbosity, never again will you be forced to type the 8 characters of the word “function,” like some servile code monkey. You don't suffer edge cases gladly. For every idiosyncrasy of ES5, you have a countermeasure.
Do not be overwhelmed by this reality. Think of ES5 as the rich, storied landscape, whose crumbling arcades and pockmarked boulevards are indicative not of disarray, but of culture and history. And ES6 is the dope fleet of Segways with which you and your hip tourist friends can cruise around, hopping between bars and restaurants without getting your shoes muddied by any of said history. ☹️
We will layer in ES6 features where they make sense, but if you understand ES5 first, then you will be delighted by the ES6 improvements, rather than being overwhelmed by too many options. However, possessing a high-level awareness of the differences can be helpful. ES6 Features is a great site showing off the new features as well as how to implement them in ES5. Don't memorize all of these—and certainly do not feel the need to understand everything—but if you end up struggling with something in ES5, you may remember that there’s an ES6 solution. And that’s when you should see if there’s a nicer ES6 implementation.
While one could simply learn ES6 exclusively, making sure to always transpile one’s code, you probably would have a hard time. Many code examples on the web are not written in ES6 syntax, and even then, ES6 is mostly new features or shortcuts, so you’d still need to know the old way of doing things. So we’re going to take a parallel approach, teaching you ES5 first and sprinkling in some ES6 knowledge when it is truly useful or when it will make your life easier. The difference is really an illusion, ES6 is just another layer of history on the great palimpsest that is the ECMAScript spec.