const approximate_pi = n => {
  let pi = 0
  let denom = 1
  for(let i = 0;i<n;i++){
    pi += (i%2?-4:4)/denom
    denom += 2
  }
  return pi
}

let dec = 10
console.log(`100 turns:    `,approximate_pi(100).toFixed(dec))
console.log(`1000 turns:   `,approximate_pi(1000).toFixed(dec))
console.log(`10000 turns:  `,approximate_pi(10000).toFixed(dec))
console.log(`100000 turns: `,approximate_pi(100000).toFixed(dec))