Quick and dirty sine calculator

sine.js

// JS implementation of series definition:
// https://en.wikipedia.org/wiki/Sine#Series_definition

function sine (x, iterations = 100) {
  let v = 0
  for (let i = 0; i < iterations; i++) {
    v += (Math.pow(-1, i)/factorial((2 * i) + 1)) * Math.pow(x, (2 * i) + 1) 
  }
  return v
}

// alternative implementation
function sine2 (x, iterations = 100) {
  let v = x
  let sign = -1
  for (let i = 3; i < iterations; i+=2) {
    v += sign * (Math.pow(x, i)/factorial(i))
    sign = -sign
  }
  return v 
}

// performance killer
function factorial (x) {
  if (x === 1) return x
  return x * factorial(x - 1)
}