InDesign script: change Page in placed PDF (selection)

SelPlacePDFPage.jsx

;(function selPlacePDFPage(  dt,doc,sel,t,ok,ff)
//----------------------------------
// Simple interface for changing page in placed PDF.
// (Just select the PDF container and run.)
{
	dt = +new Date;
	try
	{
		doc = app.properties.activeDocument;
		if( !doc ) throw "No active document.";

		sel = doc.properties.selection;
		if( !(sel && sel.length) ) throw "No selection.";
		if( !(sel=sel[0]).isValid ) throw "Invalid selection.";

		for( t=sel ; 'PDF' != t.constructor.name ; t=t[0] )
		{
			ok = t.hasOwnProperty('contentType')
			&& ContentType.graphicType == t.properties.contentType
			&& 1 == (t=t.properties.allGraphics||[]).length;
			if( !ok ) throw "The selection has no direct PDF content.";
		}
		
		ff = (t=t.properties.itemLink||0).isValid && (t.properties.filePath||0);
		if( !(ff && File(ff).exists) ) throw "PDF File is missing. You probably should relink.";

		// Runtime error if canceled by the user (hence the `dt` timer.)
		// ---
		sel.place(ff, true);
		
		alert( "PDF page sucessfully placed!" );
	}
	catch( e )
	{
		dt = +new Date - dt;        // full delay, in ms.
		if( 500 > dt ) alert( e );  // Only display instant error.
	}
})();