Exemple how u can see this data sending, with key/value exemple:
g_game.sendExtendedOpcode(SPELLUPGRADE_OPCODE, json.encode({
spell = "Mass Healing",
maxLevel = 10,
storage = 49998,
requiredItems = {
{id = 6547, count = 5},
{id = 6549, count = 2}
}...