Spend a half decade writing Perl and you'll develop an eye for minute details like that.
People aren't kidding when they say Perl looks like line noise, but you get used to it and can find your way through even a dizzying regular expression with relative ease over time.
You can get used to it, but if it takes 5 years to get reliable at parsing line noise then that many more programmers are struggling with it at any given time. Why shouldn't languages try to help rather than hinder for 5 years?
C++, by way of example, is extremely complicated and wickedly unforgiving. JavaScript, even at its most surreal, is nowhere near that.
Suggesting that things like => are "too confusing" for JavaScript is to suggest that people that write JavaScript are too dumb to handle it. Is that what you're implying here?
Some programmers write in one language, but many write in several. Having the same shorthand available in JavaScript as in C# is a big deal, it makes things feel better. Being able to apply what you've learned in one domain directly to another is more efficient than having some ridiculously quirky JavaScript-only way of doing something.
That some people will abuse it or be confused is a problem that can be overcome with a little bit of learning. It's not a big deal.
People aren't kidding when they say Perl looks like line noise, but you get used to it and can find your way through even a dizzying regular expression with relative ease over time.
Yes, $s=~s+s+S+s+$s is valid Perl.