Факториал числа JS

в чём вопрос?

ладно, давай немного разнообразия

https://jsfiddle.net/yLat973x/

function factorial(n) {
return _factorial(n, primes(n))
}

function _factorial(n, primes) {
if (n < 2) return 1

const factorialNumber = _factorial(Math.floor(n / 2), primes)
const primeSwingNumber = primeSwing(n, primes)

return factorialNumber ** 2 * primeSwingNumber
}

function primeSwing(n, primes) {
const multipliers = []

for (let i = 0; i < primes.length && primes[i] <= n; i++) {
const prime = primes[i]

let p = 1

let q = n

while (q !== 0) {
q = Math.floor(q / prime)

if (q % 2 === 1) {
p = p * prime
}
}

if (p !== 1) {
multipliers.push(p)
}
}

return product(multipliers, 0, multipliers.length - 1)
}

function product(multipliers, i, j) {
if (i > j) return 1
if (i === j) return multipliers[i]

let k = (i + j) >>> 1

return product(multipliers, i, k) * product(multipliers, k + 1, j)
}

function primes(upTo) {
upTo++

if (upTo >= 0 && upTo < 3) return []

let length = upTo >>> 1

const sieve_bool = Array.from({
length
}, () => false)

for (let i = 1, iterations = Math.sqrt(length - 1); i < iterations; i++) {
if (!sieve_bool[i]) {
for (let step = 2 * i + 1, j = i * (step + 1); j < length; j += step) {
sieve_bool[j] = true
}
}
}

let not_primes = 0

for (let not_prime of sieve_bool) {
if (not_prime) not_primes++
}

let sieve_int = Array(length - not_primes)

sieve_int[0] = 2

for (let i = 1, j = 1; i < length; i++) {
if (!sieve_bool[i]) {
sieve_int[j++] = 2 * i + 1
}
}

return sieve_int
}