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Generate a TxID using AvalancheJS

Introduction​

A transaction id, or TxID, is a string of numbers and letters which identifies a specific transaction on the blockchain. TxIDs are one of the most core components that developers interact with when working on a blockchain system. They're deterministic and can be generated using AvalancheGo or AvalancheJS.

On the Avalanche network a TxID is a CB58 encoded string which is created by sha256 hashing the transaction. CB58 is a base58 encoding with a checksum. Below are the steps for deterministically generating a TxID using AvalancheJS.

Creating a Signed Transaction​

The script which we're using can be found in the AvalancheJS example scripts as example/avm/baseTx-avax.ts. First, create a BaseTx. We're omitting that part for the sake of brevity. Once you have a BaseTx then create an UnsignedTx and sign it. Lastly convert the Tx to a Buffer by calling tx.toBuffer().

// Manually build BaseTx via steps in example/avm/baseTx-avax.ts
// Create an UnsignedTx with the BaseTx
const unsignedTx: UnsignedTx = new UnsignedTx(baseTx)
// Sign the UnsignedTx to create a Tx
const tx: Tx = unsignedTx.sign(xKeychain)
// Convert the Tx to a Buffer
const txBuf: Buffer = tx.toBuffer()

Generate the TxID​

Next, create a sha256 hash of the Buffer from the previous step.

// Create sha256 hash of the Tx Buffer
const sha256Hash: Buffer = Buffer.from(
  createHash("sha256").update(txBuf).digest().buffer
)

As mentioned in the Introduction, a TxID is a CB58 encoded string which is created by sha256 hashing the transaction. To create the TxID now CB58 encode the newly created sha256 hash.

// cb58 the sha256 hash
const generatedTxID: string = bintools.cb58Encode(sha256Hash)
console.log(`Generated TxID: ${generatedTxID}`)

The generatedTxID will be a CB58 encoded string similar to eLXEKFFMgGmK7ZLokCFjppdBfGy5hDuRqh5uJVyXXPaRErpAX.

Confirm TxID is Correct​

To confirm that the generatedTxID is correct issue the BaseTx to AvalancheGo and compare the TxID which is returned with the recently created TxID.

// get the actual txID from the full node
const actualTxID: string = await xchain.issueTx(tx)
console.log(`Success! TxID: ${actualTxID}`)

// Note the generated TxID and the returned TxID match
Generated TXID: eLXEKFFMgGmK7ZLokCFjppdBfGy5hDuRqh5uJVyXXPaRErpAX
Returned TXID: eLXEKFFMgGmK7ZLokCFjppdBfGy5hDuRqh5uJVyXXPaRErpAX

Summary​

TxIDs are a core component of any blockchain system. They are used extensively in Avalanche when creating transactions, issuing new assets and even spinning up Subnets and validators. TxIDs are deterministically created by sha256 hashing a Buffer of the transaction and then CB58 encoding the hash.