Tengo una función para Project Euler #7. Hacia el final, cambié el código de primeArray.push(i); a primeArray.unshift(i) y return primeArray[primeArray.length - 1]; para return primeArray[0]; . Esto alteró el regreso. En la antigua. Devolvió la respuesta correcta, 104021, mientras que el último devolvió 20001, que ni siquiera es primo. No entiendo por qué es eso.

 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));