Generate Item Code Based On Custom Logic

 cur_frm.cscript.customvalidate = function (doc) {
  // clear itemcode (name is from itemcode)
  doc.itemcode = "";

  // first 2 characters based on itemgroup
  switch (doc.itemgroup) {
    case "Test A":
      doc.itemcode = "TA";
      break;
    case "Test B":
      doc.itemcode = "TB";
      break;
    default:
      doc.itemcode = "XX";
  }

  // add next 2 characters based on brand
  switch (doc.brand) {
    case "Brand A":
      doc.itemcode += "BA";
      break;
    case "Brand B":
      doc.itemcode += "BB";
      break;
    default:
      doc.itemcode += "BX";
  }
};