Indesign Script - Get pageitem stacking index & select item based on the same ruleset.jsx

var selection = app.selection[0];
var zIndex = getStackingIndex( selection, true );
$.writeln( zIndex );
// Since the last parameter is set to false, this selectPageItem() mirrors the selection
// - If first item is selected when the script is run, the last one will be 
//    selected when the script is done.
selectPageItem( selection.parentPage, zIndex, false ); 




// cssMode reverses the order → Frontmost item is the highest number.
function getStackingIndex( pageItem, cssMode ) {

  var page = pageItem.parentPage;
  var pageItems = page.allPageItems;
  if ( cssMode ) pageItems = pageItems.reverse();
  var length = pageItems.length;

  for ( var i=0; i < length; i++) {
    var loopItem = pageItems[i];
    if ( pageItem === loopItem ) {
      pageItem = i;
      break;
    }
  }
  
  return pageItem;

}

// cssMode reverses the order → Frontmost item is the highest number.
function selectPageItem( page, index, cssMode ) {

  if ( cssMode ) {
    var reversedItems = page.allPageItems.reverse();
    app.selection = reversedItems[ index ];
  }
  else {
    app.selection = page.allPageItems[ index ];
  }
    
}