Did you bench it. It looks that hashset is not required or is slower that this solution. Probable because L3 cache

I like your solution, just use strings.Builder instead concatenaning strings

The kata is OK, the test is not extreme

Javascript version extreme test is not so extreme. My solution pass with 5000 characters when others will timeout. 60 chars is very tiny

Feedback: SRP over DRY

87 104 121 32 117 115 101 32 100 101 99 105 109 97 108 32 110 117 109 98 101 114 115 ?

find but may stop earley, for example "[][][][][][][][][][][][][][][]"

avoid global vars. Apart of this is ok

+1 Comment your code please :(

My solution which is a linear gets timeout several times just because stdout. Probably you may reduce a couple of tests. Any non linear solution will fail the performance 1st test.

¡I can't believe that this pass kata performance tests?

possible Liskov substitution principle violation

Exelent sieve. You may compress a range using your sieve instead an enumeration but is OK

why 12_000_000 if you know 'n'