How to get transaction hash without waiting for await call?

Question

for my demo I need the transaction hash instantly without waiting for callback of sync call. Here my code:

aggiungiAllaBlockChain: async(u,n,c,e) => {

try {
  const web3     = new Web3(App.web3Provider);
  const contract_PartecipazioneContest = new web3.eth.Contract(JSON.parse(App.ABI_PartecipazioneContest), App.ADDR_PartecipazioneContest);
  const account  = App.account;

  //const receipt  = await App.sendUserInBlockChain(web3, account, contract_PartecipazioneContest.methods.addUser(u, n, c, e));
  
  const transaction = contract_PartecipazioneContest.methods.addUser(u, n, c, e);
  value = 0;

  const options = {
      to      : transaction._parent._address,
      data    : transaction.encodeABI(),
      gas     : await transaction.estimateGas({from: account.address, value: value}),
      gasPrice: await web3.eth.getGasPrice(),
      value   : value
  };
  const signed  = await web3.eth.accounts.signTransaction(options, account.privateKey);
  const receipt = await web3.eth.sendSignedTransaction(signed.rawTransaction)
  .on('transactionHash', function(hash){
      console.log("hash on(transactionHash " + hash);
      return hash;

  });


  
} catch (error) {

  console.log("errore Aggiungi utente alla blockchain" + error)
}
}

I print the hash of my transaction but the return value doesn't work.

I have got this result using truffle but now I need this following this path (not passing for meta mask)

Thanks a lot for your help.

Answer 1

You can execute sendSignedTransaction using the event emitter instead of using the promise.

See this coding example from the official documentation.

It is given for sendTransaction, but it works the same for sendSignedTransaction.

---

BTW, you may want to extend this:

await transaction.estimateGas({from: account.address})

To this:

await transaction.estimateGas({from: account.address, value: value})

Answer 2

You can precalculate a transaction hash with all the transaction parameters and transaction signature. The transaction hash is the Keccak256 hash of the signed transaction: Keccak256(RLP(nonce, gasPrice, gasLimit, to, value, data, v, r, s)). Here's an example of how you could do it with Web3 and rlp.

import { encode } from 'rlp';

const transaction = {
  // Transaction parameters
  to: transaction._parent._address,
  gasPrice: await web3.eth.getGasPrice(),
  [...],

  // Signature
  v: ...,
  r: ...,
  s: ...
};

const rawTransaction = encode([transaction.nonce, transaction.gasPrice, transaction.gasLimit, transaction.to, transaction.value, transaction.data, transaction.v, transaction.r, transaction.s]);
const transactionHash = web3.sha3(rawTransaction);

console.log(transactionHash);