Get inset

function getInset(rectA, rectB) {
    return {
      top: (rectB.top - rectB.top),
      right: (rectA.left + rectA.width) - (rectB.left + rectB.width),
      bottom: (rectA.top + rectA.height) - (rectB.top + rectB.height),
      left: rectB.left - rectB.left
    };
  }