block13.js

function _isViewable(
  viewAreaMode: boolean,
  viewablePercentThreshold: number,
  top: number,
  bottom: number,
  viewportHeight: number,
  itemLength: number,
): boolean {
  if (_isEntirelyVisible(top, bottom, viewportHeight)) {
    // Entirely visible 
    return true;
  } else {
    // Get viewable height of this item cell 
    const pixels = _getPixelsVisible(top, bottom, viewportHeight);
    // Get the viewable percentage of this item cell 
    const percent =
      100 * (viewAreaMode ? pixels / viewportHeight : pixels / itemLength);
    return percent >= viewablePercentThreshold;
  }
}

function _getPixelsVisible(
  top: number,
  bottom: number,
  viewportHeight: number,
): number {
  const visibleHeight = Math.min(bottom, viewportHeight) - Math.max(top, 0);
  return Math.max(0, visibleHeight);
}

function _isEntirelyVisible(
  top: number,
  bottom: number,
  viewportHeight: number,
): boolean {
  return top >= 0 && bottom <= viewportHeight && bottom > top;
}