How to get raw payload from Polkadot JS transaction

Question

As the title asks, how do we abstract the raw payload of a transaction created with Polkadot JS?

// set up a tx, e.g. join a nomination pool
const tx = api.tx.nominationPools.join(bond, poolId);

// get raw payload data to sign elsewhere
// tx.???

I guess it should return something similar to the createSigningPayload mentioned in this wiki (very outdated): https://wiki.polkadot.network/docs/build-transaction-construction

so far...

Doing the following creates a bare-bones extrinsic with empty block hash / genesis, and has incomplete data compared to a payload submitted on JS Apps:

const extrinsicPayload = api.registry.createType('ExtrinsicPayload', tx, {
   version: tx.version,
});

I've determined that each payload property needs to be injected into the above example. I will update to a working example shortly.

Answer 1

Solution: The payload needs to be populated and formatted manually. Here is a complete example:

Notes:

• era is immortal, meaning the tx will always be valid.
• method just requires a Call type of your api.query...
• from is the signer / sender address.
• Haven't tested this with batching.

const lastHeader = await api.rpc.chain.getHeader();

const nonce = (await api.system.account(from)).nonce.toNumber()

const payload = {
  specVersion: api.runtimeVersion.specVersion.toHex(),
  transactionVersion: api.runtimeVersion.transactionVersion.toHex(),
  address: from,
  blockHash: lastHeader.hash.toHex(),
  blockNumber: lastHeader.number.toHex(),
  era: '0x00',
  genesisHash: api.genesisHash.toHex(),
  method: api.createType('Call', tx).toHex(),
  nonce,
  signedExtensions: Object.values(
    api.registry.metadata.extrinsic.signedExtensions.toHuman() || {}
  )?.map((e: any) => e.identifier),
  tip: tx.tip.toHex(),
  version: tx.version,
};

const raw = api.registry.createType('ExtrinsicPayload', payload);