code: function getSentinelPosition(stickyElement, sentinel, className) {
    const stickyStyle = window.getComputedStyle(stickyElement);
    const parentStyle = window.getComputedStyle(stickyElement.parentElement);
  
    switch (className) {
      case ClassName.SENTINEL_TOP:
        return {
          top: `calc(${stickyStyle.getPropertyValue('top')} * -1)`,
          height: 0,
        };
  
      case ClassName.SENTINEL_BOTTOM:
        const parentPadding = parseInt(parentStyle.paddingTop);
  
        return {
          bottom: stickyStyle.top,
          height: `${stickyElement.getBoundingClientRect().height + parentPadding}px`,
        };
    }
  }
docs: Determine the position of the sentinel
  
  @param {Element|Node} stickyElement
  @param {Element|Node} sentinel
  @param {String} className
  @returns {Object}