My code for script - next_id is a button defined on the form. I modified your code to return any records (like '%') and to simply assign this value to the new value without any manipulation. I still get a blank though. Any further ideas?

gistfile1.txt

cur_frm.cscript.next_id = function(doc)
    {
    doc.item_code = "";

    switch(doc.item_group)
        {
        case "100-XXXXXX - RESISTOR":
            doc.item_code = "100-";
            break;
        case "110-XXXXXX - CAPACITOR":
            doc.item_code = "110-";
            break;
        default:
            doc.item_code = "010-";
        }

  wn.call(
    {
    method: "webnotes.widgets.query_builder.runquery", 
    args:
        {
        "query":"SELECT `tabItem`.name \
          FROM `tabItem` WHERE `tabItem`.name like '%' \
          ORDER BY name desc limit 1"
        }, 
    callback: function(r)
        { 
        // returned last value
        last_id = r.values[0][0]
    
        var new_id = last_id;
    
        cur_frm.set_value("item_code", new_id);
        }
    });

    //doc.item_code += "000000";
  refresh_field("item_code");
    }