u1 = (1 / 1!) * (1!)
u2 = (1 / 2!) * (1! + 2!)
u3 = (1 / 3!) * (1! + 2! + 3!)
...
un = (1 / n!) * (1! + 2! + 3! + ... + n!)

let sums = [];
...
for (...) {
    sums[i] = sums[i-1] + fac(i);
}
...
return (1 / fac(i)) * sums[n]

const a = performance.now();
un2(20000000000);

// 0.004999979864805937
console.log(performance.now() - a);

const un2 = n => {
  let r = 1;

  for (let i = n, t = 1; i > 1 && i > (n - 10); i--) {
    r += 1 / (t *= i);
  }
  
  return r;
};