wow all same idea :P plus all variables as well

Because you don't have to test for anything above 1/2 the number... 2 is the lowest divisible number to check for. Saves computing time.

By accounting for lines with mutliple markers, right?

How about allowing markers to be more than 1 character in length to increase the difficulty?

It also fails for '1.2.3.4.z'

This kata (javascript) has way too few tests. Far too many incorrect solutions are being submitted.

Yes, except your variables are not declared locally. Also, there is no need to javascript-escape $... '\$' === '$'

why not just filter(x => !set.has(x))

num / 2 ? why? do you mean sqrt(num) ?