Initial Solution Setup:

var primeArray = function(n) {
  var primeArray = [];
  // fill it with primes up to 'n'
  return primeArray;
}

I don't think overwriting its own function is a good practice.

• Ruby 3.0 should be enabled (Refer this & this for more detail)

b=0 should result in NaN, not undefined. That's how modulo works everywhere.

Needs a static test case for a and b having a different sign and the expected modulo being zero.

The random tests do not always catch this scenario, and it's a possible failure mode for a certain naive solution.