solana web3 API: how to obtain data created with splToken.createTransferInstruction() - solana

Question

Would appreciate guidance on how to obtain the data back in the variables, which was entered into splToken.createTransferInstruction(), step-by-step, from the buffer variable in the code snippet below:
var transaction = new web3.Transaction();
transaction.add(
splToken.createTransferInstruction(
fromTokenAccount,
toTokenAccount,
senderPublicKey,
amount,
[],
splToken.TOKEN_PROGRAM_ID,
)
);
// Setting the variables for the transaction
transaction.feePayer = provider.publicKey;
let blockhashObj = await connection.getRecentBlockhash();
transaction.recentBlockhash = await blockhashObj.blockhash;
// Transaction constructor initialized successfully
if(transaction) { console.log('Txn created successfully'); }
// Request creator to sign the transaction (allow the transaction)
let signed = await provider.signTransaction(transaction);
let buffer = signed.serialize();
Using web3.Transaction.from(buffer) I obtained a Transaction object - see image from the browser's console:
I need to do something with instructions[0].data, I suppose to break it into byte lengths that will allow me to repopulate from the signed transaction:
fromTokenAccount,
toTokenAccount,
senderPublicKey,
amount,
[], // would this be a variable or zero-byte?
splToken.TOKEN_PROGRAM_ID,
from the splToken.createTransferInstruction() above.
Much appreciated!

Answer 1

From the TransactionInstruction, you can use decodeTransferInstruction to get back the initial parameters. In your case, you can call:
let tx = web3.Transaction.from(buffer);
let decodedIx = decodeTransferInstruction(tx.instructions[0]);
console.log(decodedIx.keys.source);
console.log(decodedIx.keys.destination);
console.log(decodedIx.keys.owner);
console.log(decodedIx.data.amount);
Full source code available at: https://github.com/solana-labs/solana-program-library/blob/24baf875e9e19c26d694d28c557d33848c3a9180/token/js/src/instructions/transfer.ts#L87