JS CS4 splitting the document path

Question

I'm trying to split the document file path so I may navigate up one or two folders and then into a specific subfolder.
When I try to split the "myDoc.filePath" it errors. If I split "myDoc.name" it works correctly.
var myDoc = app.activeDocument;
var myDocPath = myDoc.filePath;
var mySplit = myDoc.name.indexOf ("_");
var myFileName = myDoc.name.slice (0,mySplit);
var myPdfFolder = myFileName + "pdf";
var mySavePath = myDocPath
var mySplit2 = myDoc.filePath.indexOf (myFileName);
var myFinalPath = myDoc.filePath.slice (0,mySplit2) + myFileName + "/" + myPdfFolder;
alert(myFinalPath);
Can someone shed light on how to split the document path?
Thanks,
John
Document path: ~desktop/N123456/working/N123456_01
Note: the last folder name does not always exist. Often the "working" folder is the last folder. Working folder and N123456pdf reside on the same level.
Goal is to change the save to document path: ~desktop/N123456/N123456pdf

Answer 1

Here's some code I wrote recently to find a folder named "Snippets" starting from the location of a document. It first looks in the same folder and then its parent and on up until it reaches the top of the file hierarchy:
//DESCRIPTION: Search for Snippets folder from Document location
(function() {
    if (app.documents.length > 0) {
        var myFolder = locateRelatedFolder(app.documents[0].filePath, "Snippets");
        if (myFolder == null) {
            alert("Unable to locate Snippets folder");
            return;
        } else {
            alert("Snippets folder is: " + myFolder);
    function locateRelatedFolder(folder, name) {
        while (folder != null) {
            var relatedFolder = Folder(folder + "/" + name);
            if (relatedFolder.exists) return relatedFolder;
            folder = folder.parent;
    } // end locateRelatedFolder
I've enclosed the code in an anonymous function. Don't let that put you off. You could just delete the first line after the description and the last line.
Dave