Batch Replace Smart Objects and Save

Im currently using a script to do batch mockups. However it currently saves the file as 

Original PSD Name + Photo.jpg

Original PSD Name = "Mockup"

Photo = "P1"

Output = "Mockup_P1.jpg"

How do I make it so it's P1_Mockup.jpg as the output instead?

// Replace SmartObject’s Content and Save as JPG
// 2017, use it at your own risk
// Via @10354895 B: https://graphicdesign.stackexchange.com/questions/92796/replacing-a-smart-object-in-bulk-with-photoshops-variable-data-or-scripts/93359
// JPG code from here: https://forums.adobe.com/thread/737789

#target photoshop
if (app.documents.length > 0) {
    var myDocument = app.activeDocument;
    var theName = myDocument.name.match(/(.*)\.[^\.]+$/)[1];
    var thePath = myDocument.path;
    var theLayer = myDocument.activeLayer;
    // JPG Options;
    jpgSaveOptions = new JPEGSaveOptions();  
    jpgSaveOptions.embedColorProfile = true;  
    jpgSaveOptions.formatOptions = FormatOptions.STANDARDBASELINE;  
    jpgSaveOptions.matte = MatteType.NONE;  
    jpgSaveOptions.quality = 12;   
    // Check if layer is SmartObject;
    if (theLayer.kind != "LayerKind.SMARTOBJECT") {
        alert("selected layer is not a smart object")
    } else {
        // Select Files;
        if ($.os.search(/windows/i) != -1) {
            var theFiles = File.openDialog("please select files", "*.psd;*.tif;*.jpg", true)
        } else {
            var theFiles = File.openDialog("please select files", getFiles, true)
        };
        if (theFiles) {
            for (var m = 0; m < theFiles.length; m++) {
                // Replace SmartObject
                theLayer = replaceContents(theFiles[m], theLayer);
                var theNewName = theFiles[m].name.match(/(.*)\.[^\.]+$/)[1];
                // Save JPG
                myDocument.saveAs((new File(thePath + "/" + theName + "_" + theNewName + ".jpg")), jpgSaveOptions, true,Extension.LOWERCASE);
            }
        }
    }
};
// Get PSDs, TIFs and JPGs from files
function getFiles(theFile) {
    if (theFile.name.match(/\.(psd|tif|jpg)$/i) != null || theFile.constructor.name == "Folder") {
        return true
    };
};
// Replace SmartObject Contents
function replaceContents(newFile, theSO) {
    app.activeDocument.activeLayer = theSO;
    // =======================================================
    var idplacedLayerReplaceContents = stringIDToTypeID("placedLayerReplaceContents");
    var desc3 = new ActionDescriptor();
    var idnull = charIDToTypeID("null");
    desc3.putPath(idnull, new File(newFile));
    var idPgNm = charIDToTypeID("PgNm");
    desc3.putInteger(idPgNm, 1);
    executeAction(idplacedLayerReplaceContents, desc3, DialogModes.NO);
    return app.activeDocument.activeLayer
};