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Everyone's capacity to pick up a new skill is going to be different. IMO 20 hours is not enough to be proficient in any language, let alone JS. I would completely ignore a resume that said anything like 20 hours of education.
Learning / getting good at a programming language is a life style. You don't need to live and breath it, but you need to work at learning it day in day out. This is why it tends to be easier to learn things on the job. You have a built in 40 hours / week commitment to learn it.
Further - learning a language doesn't mean you're good at it. Anyone can rattle off methods and functions built into a language. What makes a good programmer is the ability to take those things you learned and build stuff or talk intelligently about it. When the boss asks "how would you approach X problem" the difference between answers from someone with 20 hours and 10,000 hours of experience will be night and day.
Good programmers bring their years of experience with them to a problem / job and 20 or 100 hours just won't cut it. Being a problem solver is so much more important than memorizing the syntax for a language.
I agree with Mario Giambanco and Emil Moe here. Learning language syntax is only a tiny fraction of being a modern-day developer.
Let's say that I believe that you actually learned the entire language of JavaScript in 20 hours total. By the way, this would never happen, but let's just say it did.
Even then, as an interviewer, I would be wondering:
1.) Does this person know their way around version control systems like Git, SVN, GitHub, GitLab, etc...
2.) Can this person present their ideas well to others?
3.) Can this person adhere to deadlines and are they an efficient worker?
4.) Is this person passionate about the subject or just trying to collect a paycheck?
Newbies often underestimate the amount that the "non-coding" skills matter. Do you know software build systems like maven, cargo, autotools, CMake, make, etc...? Do you know how to properly configure an IDE and write unit tests? Are you comfortable getting up and leading a meeting or discussion?
There are SO many things to being a good software developer that matter and that hiring managers are looking for, that even if you were able to learn an entire language in just 20 hours, you would still need much more time to learn.
That all being said, you can learn 1 subject in a language in 20 hours and maybe mediocre at best. I guarantee you if you spent all week learning about pointers in C this week, in 1 year from now, you would still learn something you didnt know about pointers even after a week of study.
All that being said, yes, I agree that 20 hours of time per subject is enough time to get your feet wet and start using that construct when programing.
Cheers, Todd