photoshop openNext

currentNameWithExtension = activeDocument.name.toString();
var fileNameNoExtension = currentNameWithExtension;
fileNameNoExtension = fileNameNoExtension.split( "." );
if ( fileNameNoExtension.length > 1 ) {
    fileNameNoExtension.length--;
}
var currentName = fileNameNoExtension;
var cutNumbers = fileNameNoExtension.toString();
cutNumbers = cutNumbers.split( "_" );

//alert("Filename is: " + currentName);
//alert("String split is: " + cutNumbers[0] + ", " + cutNumbers[1] + ", " + cutNumbers[2] + ", " + cutNumbers[3] + ", " + cutNumbers[4]);

var preNum = cutNumbers[0] + "_" + cutNumbers[1] + "_" + cutNumbers[2] + "_" + cutNumbers[3] + "_";
var postNum = cutNumbers[4];

//alert("preNum is: " + preNum + ". postNum is: " + postNum);

var FileNum = Number(postNum);
var fileNumStr = FileNum;
if ( FileNum < 100 && FileNum > 9 ) {
    fileNumStr = "0" + FileNum;
}
else if ( FileNum < 10 ) {
    fileNumStr = "00" + FileNum;
}
else {
    fileNumStr = FileNum;
}
//alert(FileNum + " " + fileNumStr);

var thisDocument = activeDocument;
var filePath = thisDocument.path;
//alert(filePath);

var openFileAt = filePath + "/";
var nextFileNum = FileNum += 1;
var nextFileNumStr = nextFileNum;
if ( nextFileNum < 100 && nextFileNum > 9 ) {
    nextFileNumStr = "0" + nextFileNum;
}
else if ( nextFileNum < 10 ) {
    nextFileNumStr = "00" + nextFileNum;
}
else {
    nextFileNumStr = nextFileNum;
}

var nextFileFull = openFileAt + preNum + nextFileNumStr + ".png";
//alert("Next file is: " + nextFileFull);

activeDocument.close(SaveOptions.DONOTSAVECHANGES);
app.open( new File ( nextFileFull ) );