m (integer > 2) which gives the start of the search (m inclusive)

n (integer >= m) which gives the end of the search (n inclusive)

That's in the kata's description, so (1, 1, 10) is not a valid input

if you want to get prime numbers with 1 gap between 1 and 10 ( "gap(1, 1, 10)" ) it should return [2,3] but it will return [1,2] and 1 is not a prime number. So you should add another verification in the isPrime function whereas if i == 1 return false

Spoiler flag, please.

We can change
j < i
to
Math.sqrt(i) + 1
in isPrime method.

This is indeed correct, thanks for noticing! I've amended my code to recognise 1 as not prime and include 4 in the prime checks. This is submitted as a new solution here:
https://www.codewars.com/kata/reviews/561eaaf5353ce442ad00004d/groups/5b43601cff888b9e620030d6

Unfortunately, I don't know how to change this particular code here...

The isPrime method recogniyes 1 and 4 as prime numbers.

Adds one space character after the element unless its the last element. But this solution is invalid anyway. It doesn't return pairs but single Strings instead with the comma at the right place so it passes the tests which unfortunately make use of Arrays.deepToString instead of comparing the array contents directly.

Hi, I undstand everyline but these
if(j != arr.length - 1)
a[i][0] += " ";

can anyone explain it ?