ExtraLongFactorial.js

function add(str1, str2) {

    let sum = "";
    let str1Length = str1.length;
    let str2Length = str2.length;

    // if s2 is longer than s1, swap them.
    if(str2Length > str1Length ){
        let temp = str2;
        str2 = str1;
        str1 = temp;
    }

    let carry = 0;
    let a;
    let b;
    let temp;
    let digitSum;
    for (let i = 0; i < str1.length; i++) {
        a = parseInt(str1.charAt(str1.length - 1 - i));
        b = parseInt(str2.charAt(str2.length - 1 - i));
        b = (b) ? b : 0;
        temp = (carry + a + b).toString();
        digitSum = temp.charAt(temp.length - 1); 
        carry = parseInt(temp.substr(0, temp.length - 1));
        carry = (carry) ? carry : 0; 
        sum = (i === str1.length - 1) ? temp + sum : digitSum + sum;  
    }

    return sum;

}

function extraLongFactorials(n) {
    let fact = 1;

    for (let i = 2; i <= n; i++){

        if(Number.isSafeInteger(fact*i)){
            fact = fact * i;
        }
        else {
            let factxi = "0";
            for(let j = 0; j < i; j++){
                factxi = add(factxi,fact.toString());  
            }
            fact = factxi;
        }
    }

    return fact;
}