system_accountNextIndex

Returns the next valid transaction index (nonce) for an account.

Parameters

Parameter	Type	Required	Description
accountId	string	Yes	SS58 address or hex AccountId. Moonbase Alpha uses 20-byte AccountId; hex like ${S.accountId} works.

cURL

curl -X POST https://api-moonbase-alpha.n.dwellir.com/YOUR_API_KEY \
-H "Content-Type: application/json" \
-d '{"jsonrpc":"2.0","id":1,"method":"system_accountNextIndex","params":["0x879364b326a766a45b4b52a941f4574f7f6a6a64"]}'

JavaScript (polkadot.js)

import { ApiPromise, WsProvider } from '@polkadot/api';

async function main() {
  const api = await ApiPromise.create({ provider: new WsProvider('${S.wsBase}') });
  const nonce = await api.rpc.system.accountNextIndex('${S.accountId}');
  console.log('Next nonce:', nonce.toString());
  await api.disconnect();
}

Notes

• For Substrate signing, pass the same nonce when constructing a signed extrinsic.
• Combine with payment_queryInfo to estimate fees before submission.