Trying to use a Photoshop script to move active layer and export as PNG

Question

I am trying to create a basic animation in which a white text object is a clipping mask for a blood drip graphic that slowly moves down to fill in the text. My plan is to create multiple frames and piece them together in FinalCut as the beginning of a video. I found a script that does what I am looking to do:

Script to move layer in photoshop

I am unable to get the script to work, and am given the following error:

"Error 2: docReg is undefined

Line: 20 -> docRef.exportDocument (pngFile, ExportType.SAVEFORWEB, opts);"

Here is the js that I am using:

// Save selected layer to variable:

app.preferences.rulerUnits = Units.PIXELS;

for (var dy = 1; dy < 50 ; dy++) {

activeDocument.activeLayer.translate(0,2);


        var opts;
        opts = new ExportOptionsSaveForWeb();
        opts.format = SaveDocumentType.PNG;
        opts.PNG8 = false;
        opts.quality = 100;
        var suffix = zeroPad(dy,5)
    var pngFile = new File("/Desktop/filename"+suffix+".png");

    //Folder path for new images

    docRef.exportDocument (pngFile, ExportType.SAVEFORWEB, opts);

}

    function zeroPad(n, s) {   
    n = n.toString();   
    while (n.length < s)  n = '0' + n;   
    return n;

}; 

Thank you for your help!

Answer 1

it's cause docRef isn't defined in script

here is modified script

// Save selected layer to variable:

var fileName = prompt("Enter File Name","");

app.preferences.rulerUnits = Units.PIXELS;

for (var dy = 1; dy < 50 ; dy++) {

activeDocument.activeLayer.translate(0,2);


        var opts;
        opts = new ExportOptionsSaveForWeb();
        opts.format = SaveDocumentType.PNG;
        opts.PNG8 = false;
        opts.quality = 100;
        var suffix = zeroPad(dy,5)
    var pngFile = new File("~/Desktop/"+fileName+suffix+".png");

    //Folder path for new images

    app.activeDocument.exportDocument (pngFile, ExportType.SAVEFORWEB, opts);

}

    function zeroPad(n, s) {   
    n = n.toString();   
    while (n.length < s)  n = '0' + n;   
    return n;

};

Edit : this one will now ask you for filename