InDesign script: Prompts a warning if the inside/left slug of the opened document is less than `MIN_SLUG`

checkSlug.jsx

(function(/*any*/ev,  ff,t,cond,bk,pf,x)
//----------------------------------
// [This InDesign script CAN be used as a startup script.]
// Prompts a warning if the inside/left slug of the opened document is less than `MIN_SLUG`.
// Targeted event: afterOpen (app listener)
{
	const MIN_SLUG = 130; // in mm
	const MM = +MeasurementUnits.MILLIMETERS;

	if( (ev||0).isValid )
	{
		cond = 'afterOpen' == ev.eventType                       // Make sure you're managing the right event
		&& (!app.modalState)                                     // app.modalState would cause `alert` to fail.
		&& 'LayoutWindow'==(t=ev.target).constructor.name        // ev.target must be a LayoutWindow (not a document)
		&& (t=t.parent).isValid                                  // Valid document required.
		&& (t.properties.saved||t.properties.converted)          // Want saved or converted docs (excl. recovered and unsaved ones).
		;
		if( cond )
		{
			pf = app.scriptPreferences;
			MM == (bk=+pf.measurementUnit) ? (bkp=0) : (pf.measurementUnit=MM);
			x = parseFloat(t.documentPreferences.slugInsideOrLeftOffset);
			bk && (pf.measurementUnit=bk);
			if( x < MIN_SLUG ) alert( "The left slug area ("+x+") is less than "+MIN_SLUG+"mm." );
		}
	}
	else
	{
		// Install the event listener (if not yet installed).
		// ---
		const UID = 'myCheckSlugHandler';
		(ff=File($.fileName)).exists
		&& !((t=app.eventListeners).itemByName(UID)).isValid
		&& ((t.add('afterOpen',ff)).name=UID);
	}

})($.global.evt);