Javascript get all Layers inside a Layer Group tree

You need to use a recursive function then you do whatever you want with all the layers in the array layerList:

var doc = activeDocument;
var layerList = new Array()
getLayers(doc);

function getLayers(lSet){

    for(var i=0;i<lSet.layers.length;i++){
        doc.activeLayer = lSet.layers;

        if(doc.activeLayer.typename == 'LayerSet'){
            if(doc.activeLayer.layers.length>0){
                getLayers (doc.activeLayer)
                }
            }
        else{
            layerList.push(doc.activeLayer)
            }        
        };//end loop
    };//end function