HowTo: Construct a txID of an unsigned PSBT

Hi,

as the title says, I would like to build the txID of an unsigned PSBT transaction. I built the PSBT with the following code snippet:

const psbt = new bitcoin.Psbt({ network });
psbt.setVersion(1);
for (const input of finalInputs) {
  psbt.addInput({ hash: input.txId, index: input.vout, witnessUtxo: input.WitnessUtxo });
}
finalOutputs.forEach(output => {
   psbt.addOutput({ address: output.address, value: output.value });
});

As I read through some threads here, I saw that it’s possible to get the txID with BuildIncomplete via the TransactionBuilder but I wasn’t able to get it in any way from a PSBT transaction.

I would really appreciate if you could point me into the right direction!

[SiestaMadokaist]

you could get the TransactionBuilder via:

const psbt = new bitcoin.Psbt({ network })
// doing things with psbt.
const tx = psbt.__CACHE.__TX;
// that will give you a bitcoin.Transaction object, if you need the bitcoin.TransactionBuilder then you need to add the following line.
const txb = bitcoin.TransactionBuilder.fromTransaction(tx)

although, the __CACHE is for now still private interface, meaning, it might break in the future update.

I’m not sure if there’s a more appropriate way to do that as of now.

[junderw]

I was speaking on a theoretical basis.

In terms of your method (unsupported) the reason why this is happening is because PSBT leaves the inputs empty until the tx is extracted.

With P2SH-P2WSH you will need to have a scriptSig (input script) with the redeemscript.

tx.ins[i].script = bitcoinjs.script.compile([redeemScriptBuffer]) should allow you to use the work around for P2SH-P2WSH as well.

However, be careful, as JavaScript passes by reference, so if you add those scripts the scripts will be added to the internal TX of the PSBT as well… so don’t do anything with the PSBT object after you’ve done this.