Rectangles intersection

getIntersection.js

/**
 * Returns the intersection area (in px) based on the intersection
 * between two given rectangles (i.e. ClientRect).
 */
export default function getIntersection(rectA, rectB) {
  const intersectionX = Math.max(0, Math.min(rectA.right, rectB.right) - Math.max(rectA.left, rectB.left))
  const intersectionY = Math.max(0, Math.min(rectA.bottom, rectB.bottom) - Math.max(rectA.top, rectB.top))
  const intersectionArea = intersectionX * intersectionY
  
  return intersectionArea
}

Code origin: https://math.stackexchange.com/a/99576