Photoshop script: getSelectedLayersIndex

foo.js

var getSelectedLayersIndex = function () {
  var selectedLayers = [];
  var ref = new ActionReference();
  ref.putEnumerated(stringIDToTypeID('document'), stringIDToTypeID('ordinal'), stringIDToTypeID('targetEnum'));
  var desc = executeActionGet(ref);
  if (desc.hasKey(stringIDToTypeID('targetLayers'))) {
    desc = desc.getList(stringIDToTypeID('targetLayers'));
    for (var i = 0, c = desc.count; i < c; i++) {
      selectedLayers.push(desc.getReference(i).getIndex());
    }
  }
  return selectedLayers;
};