euler 65

let arr = [1n];
for (let i = 2n; arr.length < 99; i += 2n) {
    arr.push(i, 1n, 1n);
}
let nom = arr[98], den = 1n;
arr.unshift(2n);
const gcd = (a, b) => b ? gcd(b, a % b) : a;
for (let i = 98; i >= 0; i--) {
    [nom, den] = [den + nom * arr[i], nom];
    let g = gcd(nom, den);
    nom /= g;
    den /= g;
}
console.log((nom + "").split("").reduce((a, b) => a + parseInt(b), 0));