//the export pixel width
var targetWidthPixels = 1000;

var doc = app.activeDocument;
with(doc.viewPreferences){
    horizontalMeasurementUnits = MeasurementUnits.INCHES;
    verticalMeasurementUnits = MeasurementUnits.INCHES;
}

//get the page width in inches
var pw=doc.documentPreferences.pageWidth;

//calulate the resolution needed to export the jpeg as 1500px wide
//if the page is 8.5" x 11", the output pixel dimensions will be 600px x 776px
var resout= targetWidthPixels/pw;

//the active page
var pg="Print:3";

//set the res preference and page range
 with(app.jpegExportPreferences){
     exportResolution = resout;
     pageString = pg;
     antiAlias = true;
     embedColorProfile = true;
     jpegColorSpace = JpegColorSpaceEnum.RGB;
     //add other properties as needed
}
 
//export the JPEG
doc.exportFile(
		ExportFormat.jpg,
		File("target" + doc.name.substring(0,13) + ".jpg"), 
		false
);