react-color/src/helpers/saturation.js

export const calculateChange = (e, hsl, container) => {
  const { width: containerWidth, height: containerHeight } = container.getBoundingClientRect()
  const x = typeof e.pageX === 'number' ? e.pageX : e.touches[0].pageX
  const y = typeof e.pageY === 'number' ? e.pageY : e.touches[0].pageY
  let left = x - (container.getBoundingClientRect().left + window.pageXOffset)
  let top = y - (container.getBoundingClientRect().top + window.pageYOffset)

  if (left < 0) {
    left = 0
  } else if (left > containerWidth) {
    left = containerWidth
  }

  if (top < 0) {
    top = 0
  } else if (top > containerHeight) {
    top = containerHeight
  }

  const saturation = left / containerWidth
  const bright = 1 - (top / containerHeight)

  return {
    h: hsl.h,
    s: saturation,
    v: bright,
    a: hsl.a,
    source: 'hsv',
  }
}