.filter(Boolean) === .filter((val, index, arr) => Boolean(val, index, arr)) 
// Boolean ignores the other 2 params, so it's basically just Boolean(val)

For more info you can read the documentation for the filter function:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter

Yes - it was me who was mixed up - thanks!

Two months... i've lost two months just to find out that there is -e argument of echo command.

\n is a newline escaped sequence. The string is ok.

s = "abcd\n
     efgh\n
     ijkl\n
     mnop"

See?

The instructions example:

s = "abcd\nefgh\nijkl\nmnop"

has 'n' and 'm' mixed, it seems to me

Ok I believe this is the XOR bitwise operator, used here to filter if either group has the character in question. This has been chosen over the standard '||' as it enforces one or the other but not both.

How do you make measurements like that? I am a noob but try to think about the efficiency of my code and wonder about ways I could check.

Does anyone know the meaning of ^ in this solution?

How does the use of Boolean only filter empty strings that evaluate as false and not strings that evaluate as true?