code: 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
  }
docs: Calculate the numeric integration of a function
  @param {Function} f
  @param {number} start
  @param {number} end
  @param {number} [step=0.01]