factorial.js

function isFactorial(n) {
    let x = 0;
    let counter = 0;
    while (x <= n) {
        x = getFactorial(counter);
        if (x === n ) {
            return true;
        }
        counter++;
    }
    return false;
}

function getFactorial(n) {
    if (n >= 0) {
        if (n === 0) {
            return 1;
        }
        let x = n;
        for (i = 1; i < n; i++) {
            x = x * (n - i);
        }
        return x;
    }
}