[defatigable]

This doesn't sound right to me. If someone who were expert in JS looked at a relatively simple C++ program, I think they could reasonably well tell if the quality of code were good or not. They wouldn't be able to, e.g., detect bugs from default value initialization, memory leaks, etc. But so long as the code didn't do any crazy templating stuff they'd be able to analyze it at a rough "this algorithm seems sensible" level".

Analogously I'm quite proficient at C++, and I can easily look at a small JS program and tell if it's sensible. But if you give me even a simple React app I wouldn't be able to understand it without a lot of effort (I've had this experience...)

I agree with your broad point: C/C++/Java are certainly much more complex than JS and I would expect someone expert in them to have a much easier time picking up JS than the reverse. But given very high overlap in syntax between the four I think anyone who's proficient in one can grok the basics of the others.