function integrate (f, start, end, step) {
  let total = 0
  step = step || 0.01
  for (let x = start; x < end; x += step) {
    total += f(x + step / 2) * step
  }
  return total
}
Calculate the numeric integration of a function
@param {Function} f
@param {number} start
@param {number} end
@param {number} [step=0.01]