I have a function for Project Euler #7. Towards the end, I changed the code from primeArray.push(i); to primeArray.unshift(i) and return primeArray[primeArray.length - 1]; to return primeArray[0];. This altered the return. In the former. It returned the correct answer, 104021, while the latter returned 20001, which is not even prime. I do not understand why that is.

function primeFinder(primeTarget) {
  //since 2 is the only even prime, putting it here saves us some effort in checking primality by just iterating over the odd numbers
  //it also helps the for loop since we are only interested in checking for divisors among previous primes.
  let primeArray = [2];
  let i = 3;

  while (primeArray.length < primeTarget) {
    let primeLogger = false;
    //We don't need to check for divisibility by 2, so j can equal 1
    for (j = 1; j < primeArray.length && primeArray[j] < Math.sqrt(i); j++) {
      if (i % primeArray[j] === 0) {
        primeLogger = true;
        //Since we have found a divisor and changed primeLogger, we can exit the loop
        break;
      }
    }
    //Where the break goes to, and also where the for loop goes to once finishes
    if (primeLogger === false) {
      primeArray.push(i);
    }
    i += 2;
  }
  return primeArray[primeArray.length - 1];
}
console.log(primeFinder(10001));