Select adjacent layer on the same level.jsx

/* 
NOTE: 
  1. Makes the assumption that only one layer is selected (only really matters when going down)
  2. Doesn't care if the adjacent layer is a group or not
*/

var options = {
  direction: 'up',
  loop: true,
};

var doc = app.activeDocument;
var originalLayer = doc.activeLayer;
var originalLayerId = originalLayer.id;

var siblings = originalLayer.parent.layers;
for (var layerIndex = 0; layerIndex < siblings.length; layerIndex++) {
  
  var layer = siblings[layerIndex];
  var layerId = layer.id;
  
  // Found selected layer (again... in the array of its siblings, so we know which layers are next to it)
  if ( layerId === originalLayerId ) {
    
    // Find adjacent layer by adding or subtracting 1
    var adjacentIndex;
    if ( options.direction === 'up' ) {
      adjacentIndex = layerIndex - 1;
    } else {
      adjacentIndex = layerIndex + 1;
    }
    
    // Wrap around
    if ( options.loop ) {
      if ( adjacentIndex < 0 ) adjacentIndex = siblings.length - 1;
      if ( adjacentIndex >= siblings.length ) adjacentIndex = 0;
    }
    // Clamp to siblings length
    else {
      if ( adjacentIndex < 0 ) adjacentIndex = 0;
      if ( adjacentIndex >= siblings.length ) siblings.length - 1;
    }
    
    // Select adjacent layer if it exists
    var adjacentLayer = siblings[ adjacentIndex ];
    // if ( adjacentLayer ) doc.activeLayer = adjacentLayer;
    if ( adjacentLayer ) selectLayerById( adjacentLayer.id );
    
  }
  
}

function cTID(s) { return app.charIDToTypeID(s); };
function sTID(s) { return app.stringIDToTypeID(s); };

function selectLayerById( id ) {
  var result = false;
  try {
    var ref = new ActionReference();
    ref.putIdentifier(cTID('Lyr '), id);
    var desc = new ActionDescriptor();
    desc.putReference(cTID('null'), ref);
    // Retain layer visibility
    desc.putBoolean(sTID('makeVisible'), checkLayerStateById( id, 'visible'));
    const groupExpanded = checkLayerStateById( id, 'expanded');
    // Select layer
    executeAction(cTID('slct'), desc, DialogModes.NO);
    // Keep group expanded or not
    expandGroupById( id, groupExpanded);
    result = true;
  } catch(e) {}
  return result;
}

function checkLayerStateById( id, state ) {
  if ( state === 'expanded' ) state = 'layerSectionExpanded';
  var ref = new ActionReference();
  ref.putProperty( cTID("Prpr"), sTID( state ));
  ref.putIdentifier( cTID( "Lyr " ), id );
  try { return executeActionGet(ref).getUnitDoubleValue( sTID( state ) ) == 1 ? true : false; } catch(e) { return undefined }
}

function expandGroupById( id, expand, nested ) {
  var ref = new ActionReference();
  var desc = new ActionDescriptor();
  ref.putProperty(cTID('Prpr'), sTID('layerSectionExpanded'));
  ref.putIdentifier( cTID( "Lyr " ), id );
  desc.putBoolean(sTID('to'), !!expand);
  desc.putBoolean(sTID('recursive'), !!nested );
  desc.putReference(cTID('null'), ref);
  executeAction(cTID('setd'), desc, DialogModes.NO);
}