Nope it gives Nan

suggest adding test cases for leading whitespace and for multi-character comment markers:

checkComments("  some text with prefixed whitespace *\n   \n", ["*", "-"], "  some text with prefixed whitespace\n\n")
checkComments("comment markers can // possibly\nalso be more than one character", ["//", ")"], "comment markers can\nalso be more than one character")

Doesn't sqrt produce an error on negative numbers according to the specification?