want to apply layer mask

Why not a Vector Mask?

Anyway, this might work – it uses the first path:

// convert background layer and add layer mask from the first path;
// 2011, use it at your own risk;
#target photoshop;
if (app.documents.length > 0 && app.activeDocument.pathItems.length > 0) {
var myDocument = app.activeDocument;
var theLayer = myDocument.layers[myDocument.layers.length - 1];
if (theLayer.isBackgroundLayer == true) {theLayer.isBackgroundLayer = false};
theLayer.name = "source";
var thePath = myDocument.pathItems[myDocument.pathItems.length - 1];
thePath.makeSelection(0, true, SelectionType.REPLACE);
myDocument.activeLayer = theLayer;
// make layer mask;
var idMk = charIDToTypeID( "Mk  " );
var desc168 = new ActionDescriptor();
var idNw = charIDToTypeID( "Nw  " );
var idChnl = charIDToTypeID( "Chnl" );
desc168.putClass( idNw, idChnl );
var idAt = charIDToTypeID( "At  " );
var ref99 = new ActionReference();
var idChnl = charIDToTypeID( "Chnl" );
var idChnl = charIDToTypeID( "Chnl" );
var idMsk = charIDToTypeID( "Msk " );
ref99.putEnumerated( idChnl, idChnl, idMsk );
desc168.putReference( idAt, ref99 );
var idUsng = charIDToTypeID( "Usng" );
var idUsrM = charIDToTypeID( "UsrM" );
var idRvlS = charIDToTypeID( "RvlS" );
desc168.putEnumerated( idUsng, idUsrM, idRvlS );
executeAction( idMk, desc168, DialogModes.ALL );
};