How can you use Javascript to enter metadata into a PSD, and pull it out later?

This will place a file as a smart object then write the file's path to the exif:userComment of the layer's metadata. The it reads the layer's comment and alerts the file's path.

// these function adapted from PerLayerMetdata.jsx -Copyright 2008 Adobe Systems Incorporated
function loadXMPLibrary(){
     if ( !ExternalObject.AdobeXMPScript ){
          try{
               ExternalObject.AdobeXMPScript = new ExternalObject('lib:AdobeXMPScript');
          }catch (e){
               alert("Can't load XMP Script Library");
               return false;
          }
     }
     return true;
};
function setCommMetadata(comm){
     if( app.activeDocument.activeLayer.isBackgroundLayer){
          alert( 'Can not place metadata on a background layer.' );
     } else {
          var xmp;
          if ( comm == "" )
               comm = " ";
               
          try{
               xmp = new XMPMeta( app.activeDocument.activeLayer.xmpMetadata.rawData );
          } catch( e ) {
               xmp = new XMPMeta();
          }
          try{
               xmp.setProperty( XMPConst.NS_EXIF, "userComment", comm );
          } catch( e ) {
               alert( 'Unable to place metadata on selected layer.\n' + e );
          }
          app.activeDocument.activeLayer.xmpMetadata.rawData = xmp.serialize();
     }
};
function placeFile( file) {
    var desc = new ActionDescriptor();
    desc.putPath( charIDToTypeID('null'), file );
    desc.putEnumerated( charIDToTypeID('FTcs'), charIDToTypeID('QCSt'), charIDToTypeID('Qcsa') );
        var offsetDesc = new ActionDescriptor();
        offsetDesc.putUnitDouble( charIDToTypeID('Hrzn'), charIDToTypeID('#Pxl'), 0.000000 );
        offsetDesc.putUnitDouble( charIDToTypeID('Vrtc'), charIDToTypeID('#Pxl'), 0.000000 );
    desc.putObject( charIDToTypeID('Ofst'), charIDToTypeID('Ofst'), offsetDesc );
    executeAction( charIDToTypeID('Plc '), desc, DialogModes.NO );
};
function loadXMPLibrary(){
     if ( !ExternalObject.AdobeXMPScript ){
          try{
               ExternalObject.AdobeXMPScript = new ExternalObject
                                                            ('lib:AdobeXMPScript');
          }catch (e){
               alert( ErrStrs.XMPLIB );
               return false;
          }
     }
     return true;
};
function unloadXMPLibrary(){
   if( ExternalObject.AdobeXMPScript ) {
      try{
         ExternalObject.AdobeXMPScript.unload();
         ExternalObject.AdobeXMPScript = undefined;
      }catch (e){
         alert( ErrStrs.XMPLIB );
      }
   }
};
if( app.documents.length > 0 ){
     var imageRef = new File( "~/Desktop/DSC_5396.NEF" );
     loadXMPLibrary();
     placeFile( imageRef );
     setCommMetadata( decodeURI( imageRef ) );
     var layerXMP = new XMPMeta( app.activeDocument.activeLayer.xmpMetadata.rawData );// create the object
     var comment = layerXMP.getProperty( XMPConst.NS_EXIF, "userComment" );// get comment
}
alert( 'Orginal file is located at ' + comment );