How to get a group of units through javascript SDK?

// Print message to log
function msg(text) { $("#log").prepend(text + "<br/>"); }

function init() { // Execute after login succeed
    var sess = wialon.core.Session.getInstance(); // get instance of current Session
    // flags to specify what kind of data should be returned
    var flags = wialon.item.Item.dataFlag.base | wialon.item.Unit.dataFlag.lastMessage;

    sess.loadLibrary("itemIcon"); // load Icon Library    
    sess.updateDataFlags( // load items to current session
    [{type: "type", data: "avl_unit_group", flags: flags, mode: 0}], // Items specification
        function (code) { // updateDataFlags callback
            if (code) { msg(wialon.core.Errors.getErrorText(code)); return; } // exit if error code

            // get loaded 'avl_unit's items  
            var units = sess.getItems("avl_unit_group");
            if (!units || !units.length){ msg("Units not found"); return; } // check if units found

            for (var i = 0; i< units.length; i++){ // construct Select object using found units
                //msg("Que imprime: " + units[i]);
                var u = units[i]; // current unit in cycle
                var grupoU = u.getUnits();
                msg("Hola: " + grupoU.length);
                //for(var j=0;j<grupoU.length;j++){
                    //msg(grupoU[j]);
                //}
                //msg(grupoU);
                // append option to select
                $("#units").append("<option value='"+ u.getId() +"'>"+ u.getName()+ "</option>");
            }
            // bind action to select change event
            $("#units").change( getSelectedUnitInfo );
        }
    );
}

function getSelectedUnitInfo(){ // print information about selected Unit

    var val = $("#units").val(); // get selected unit id
    msg("Valor del grupo: " + val);
    if(!val) return; // exit if no unit selected
    
    var unit = wialon.core.Session.getInstance().getItem(val); // get unit by id
    msg(unit.getUnits());
    var unidades = unit.getUnits();
    
    msg(unidades.length);
    for(var j=0;j<inidades.length;j++){
       msg(unidades.getPosition()); //THIS PROBLEM NO INFORMATION
    }
    
    
    
    if(!unit){ 
        msg("Unit not found");
        return; 
    } // exit if unit not found
    
    // construct message with unit information
    var text = "<div>'"+unidades.getName()+"' selected. "; // get unit name
    var icon = unit.getIconUrl(32); // get unit Icon url
    if(icon) text = "<img class='icon' src='"+ icon +"' alt='icon'/>"+ text; // add icon to message
    var pos = unidades.getPosition(); // get unit position
    if(pos){ // check if position data exists
        var time = wialon.util.DateTime.formatTime(pos.t);
        text += "<b>Last message</b> "+ time +"<br/>"+ // add last message time
            "<b>Position</b> "+ pos.x+", "+pos.y +"<br/>"+ // add info about unit position
            "<b>Speed</b> "+ pos.s; // add info about unit speed
        // try to find unit location using coordinates 
        wialon.util.Gis.getLocations([{lon:pos.x, lat:pos.y}], function(code, address){ 
            if (code) { msg(wialon.core.Errors.getErrorText(code)); return; } // exit if error code
            msg(text + "<br/><b>Location of unit</b>: "+ address+"</div>"); // print message to log
        });
    } else // position data not exists, print message
        msg(text + "<br/><b>Location of unit</b>: Unknown</div>");
}

// execute when DOM ready
$(document).ready(function () {
    wialon.core.Session.getInstance().initSession("https://hst-api.wialon.com"); // init session
    // For more info about how to generate token check
    // http://sdk.wialon.com/playground/demo/app_auth_token
    wialon.core.Session.getInstance().loginToken("52e46546b26c686a90a987643f056cfcE3F6CFF45C55B3E221520C9E0FBA29D05615679D", "", // try to login
        function (code) { // login callback
            // if error code - print error message
            if (code){ msg(wialon.core.Errors.getErrorText(code)); return; }
            msg("Logged successfully"); init(); // when login suceed then run init() function
    });
});