Generate Item Code Based On Custom Logic

Add this Custom Script in the script of Item, so that the new Item Code is
generated just before the Item gets saved.

 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";
  }
};