How to confirm a transaction given only the transaction signature and slot number?

Question

I want to confirm a transaction gracefully and wait for it to be finalized. I understand the best way to do this is to use the Connection.confirmTransaction method with the BlockheightBasedTransactionConfirmationStrategy argument reference.

But that method requires 3 things, transaction signature, recent blockhash, and last valid block height. I have the first two, since recent blockhash can be obtained from the Transaction object returned from Connection.getTransaction. But how can I get the last valid block height?

Or is there any other way to handle cases like this and confirm transactions effectively?

Answer 1

But how can I get the last valid block height?

const { blockhash, lastValidBlockHeight } = await connection.getLatestBlockhash()

await connection.confirmTransaction({
  blockhash,
  lastValidBlockHeight,
  signature: "your-transaction-signature"
})

You're probably better off using the blockhash from that call rather than the one from the transaction too because it'll be more recent. Docs: https://solana-labs.github.io/solana-web3.js/classes/Connection.html#getLatestBlockhash