Yes, I did exactly what you say ,But in a more stupid way

Prototypes is workly so fast and so optimised in JS. Let go of your prejudices and explore what prototype programming is (Not only in JS).

A line break or a tab or any other whitespace character (which trims to zero-length) will break this program; the word splitting is just bare enough to pass the tests.

That's probably more future-proof way for real production code, except I'd use ES6 spread for Math.min.

While not very efficient for exactly this task; this algorithm does have the future! Image your Product Manager comes in and asks to tweak the function, to count per-language. You'd pass not only a string, but locale as well. In this case, you'd have to check the Unix index numbers of each character, because let's say Urdu could have some English characters sprinkled inside and thus void the unique character count.

redundant i regex flag

the "case insensitive" regex flag i is redundant