let summation = (start, end, a) => {
  let sum = 0;
  for (let n = start; n <= end; n++) sum += a(n);
  return sum;
}

console.log(4 * summation(0, 100000000, n => (n & 1 === 1 ? -1 : 1) / (2 * n + 1))); // 3.141592663589326