Transaction gas cost in truffle test case

Question

Below is from a truffle test cases in javascript, where I was trying to add the gas cost to an account balance to confirm the sum of transaction, where the sum should be equal to the previous balance.

But I couldn't get it to work!

The rcpt.cumulativeGasUsed, web3.eth.gasPrice, getBalance(accounts[1]) are all correct, but the arithmetic is not, some fromWei, toWei kung-fu needed?

What am I doing wrong here?

            return instance.bailOut({ from: accounts[1] }).then(function (resp) {
            var rcpt = web3.eth.getTransactionReceipt(resp.tx);
            console.log("Sum: " + ((rcpt.cumulativeGasUsed * web3.eth.gasPrice) + web3.eth.getBalance(accounts[1]).toString(10)));

});

After a lot of experiments, it looks like the below method works fine, but couldn't explain the logic of transaction cost! Any idea is most welcome.

Anyways unit test is passing now!!!

return contractInstance.withdraw({ value: web3.toWei(1, "ether"), gas: 1000000 }).then(function () {
          var contractAddressBalance = web3.fromWei(web3.eth.getBalance(contractAddress).toString(10));
          console.log("contractAddress balance after withdraw: " + contractAddressBalance);
          return contractInstance.bailOut({ from: accounts[1] }).then(function (resp) {
            var rcpt = web3.eth.getTransactionReceipt(resp.tx);

            console.log("cumulativeGasUsed: " + rcpt.cumulativeGasUsed);
            console.log("gasPrice: " + web3.eth.gasPrice);

            var transactionCost = (rcpt.cumulativeGasUsed / 10000000); // How come this works???

            console.log("transactionCost: " + transactionCost);
            console.log("Account[1] balance after withdraw: " + web3.eth.getBalance(accounts[1]));

            contractWalletAfter = web3.fromWei(web3.eth.getBalance(accounts[1]).add(web3.fromWei(rcpt.cumulativeGasUsed)));
            assert.equal(((contractWalletAfter.minus(contractWalletBefore)).add(transactionCost)).valueOf(), 6, "6 wasn't in the contract wallet");
          });
        });

Answer 1

Note: This answer is only valid for Truffle v4.

---

I've tried this test with the MetaCoin example from truffle (ie run truffle unbox metacoin in an empty directory)

var MetaCoin = artifacts.require("./MetaCoin.sol");

contract('MetaCoin', function(accounts) {
  it("Test gas", async () => {
    const meta = await MetaCoin.deployed();

    // Initial balance of the second account
    const initial = await web3.eth.getBalance(accounts[1]);
    console.log(`Initial: ${initial.toString()}`);

    // Obtain gas used from the receipt
    const receipt = await meta.sendCoin(accounts[2], 1, { from: accounts[1] });
    const gasUsed = receipt.receipt.gasUsed;
    console.log(`GasUsed: ${receipt.receipt.gasUsed}`);

    // Obtain gasPrice from the transaction
    const tx = await web3.eth.getTransaction(receipt.tx);
    const gasPrice = tx.gasPrice;
    console.log(`GasPrice: ${tx.gasPrice}`);

    // Final balance
    const final = await web3.eth.getBalance(accounts[1]);
    console.log(`Final: ${final.toString()}`);
    assert.equal(final.add(gasPrice.mul(gasUsed)).toString(), initial.toString(), "Must be equal");
  });
});

The correspoding output is:

  Contract: MetaCoin
Initial: 99971803600000000000
GasUsed: 23497
GasPrice: 100000000000
Final: 99969453900000000000
    ✓ Test gas (199ms)

  1 passing (217ms)

One difference is that I'm reading the gasPrice from the transaction (getTransaction(hash).gasPrice) instead of the network eth.gasPrice().

Answer 2

The following code is the same code as Ismael's code, with the changes to work with Truffle v5, as getBalance's return type, beside gasUsed and gasPrice values are not BN by default:

var MetaCoin = artifacts.require("./MetaCoin.sol");

const { toBN } = web3.utils;

contract('MetaCoin', function(accounts) {
  it("Test gas", async () => {
    const meta = await MetaCoin.deployed();

    // Initial balance of the second account
    const initial = toBN(await web3.eth.getBalance(accounts[1]));
    console.log(`Initial: ${initial.toString()}`);

    // Obtain gas used from the receipt
    const receipt = await meta.sendCoin(accounts[2], 1, { from: accounts[1] });
    const gasUsed = toBN(receipt.receipt.gasUsed);
    console.log(`GasUsed: ${receipt.receipt.gasUsed}`);

    // Obtain gasPrice from the transaction
    const tx = await web3.eth.getTransaction(receipt.tx);
    const gasPrice = toBN(tx.gasPrice);
    console.log(`GasPrice: ${tx.gasPrice}`);

    // Final balance
    const final = toBN(await web3.eth.getBalance(accounts[1]));
    console.log(`Final: ${final.toString()}`);
    assert.equal(final.add(gasPrice.mul(gasUsed)).toString(), initial.toString(), "Must be equal");
  });
});