Sending transactions in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ethereum transaction code snippet?

Consider this simplified Ethereum transaction sending code using web3.js. What will be printed to the console?

Blockchain / Solidity

const Web3 = require('web3');
const web3 = new Web3('https://mainnet.infura.io/v3/YOUR_PROJECT_ID');

async function sendTx() {
  const accounts = await web3.eth.getAccounts();
  console.log(accounts.length);
}
sendTx();

A 0

B An error because getAccounts is not available on mainnet without unlocked accounts

C undefined

D A positive number representing accounts length

💡 Hint

Think about what getAccounts() returns when connected to a public node without unlocked accounts.

When connected to a public node like Infura, getAccounts() returns an empty array because the node does not manage any accounts locally. So, accounts.length is 0.

❓ Predict Output

intermediate

What is the output of this Bitcoin transaction fee calculation?

Given this code snippet calculating Bitcoin transaction fee, what is the output?

Blockchain / Solidity

const txSizeBytes = 250;
const feeRateSatsPerByte = 20;
const fee = txSizeBytes * feeRateSatsPerByte;
console.log(fee);

A 250

B 270

C 20

D 5000

💡 Hint

Multiply transaction size by fee rate per byte.

The fee is calculated by multiplying the transaction size (250 bytes) by the fee rate (20 satoshis per byte), resulting in 5000 satoshis.

🔧 Debug

advanced

Why does this Ethereum transaction sending code throw an error?

Look at this code snippet that tries to send an Ethereum transaction. Why does it throw an error?

Blockchain / Solidity

const Web3 = require('web3');
const web3 = new Web3('https://mainnet.infura.io/v3/YOUR_PROJECT_ID');

async function sendTx() {
  const tx = {
    from: '0xYourAddress',
    to: '0xRecipientAddress',
    value: web3.utils.toWei('1', 'ether'),
    gas: 21000
  };
  const receipt = await web3.eth.sendTransaction(tx);
  console.log(receipt);
}
sendTx();

A Error because gas limit is too low

B Error because the private key is not provided or account is not unlocked

C Error because value is not converted to wei correctly

D No error, transaction will succeed

💡 Hint

Think about what is needed to sign and send a transaction using a public node.

When using a public node like Infura, the account is not unlocked and private keys are not managed. You must sign the transaction locally before sending. Without signing, sendTransaction throws an error.

📝 Syntax

advanced

Which option correctly creates and signs a transaction in Ethereum using ethers.js?

Choose the code snippet that correctly creates and signs a transaction using ethers.js.

A

const wallet = new ethers.Wallet(privateKey);
const tx = { to: recipient, value: ethers.utils.parseEther('1') };
const signedTx = wallet.signTransaction(tx);
console.log(signedTx);

B

const wallet = new ethers.Wallet(privateKey);
const tx = { to: recipient, value: '1' };
const signedTx = wallet.sign(tx);
console.log(signedTx);

C

const wallet = new ethers.Wallet(privateKey);
const tx = { to: recipient, value: ethers.utils.parseEther('1') };
const signedTx = await wallet.signTransaction(tx);
console.log(signedTx);

D

const wallet = new ethers.Wallet(privateKey);
const tx = { to: recipient, value: ethers.utils.parseEther('1') };
const signedTx = wallet.sign(tx);
console.log(signedTx);

💡 Hint

Check the correct method name for signing a transaction in ethers.js.

The correct method to sign a transaction in ethers.js is signTransaction. The value must be converted to wei using parseEther. Option C uses the correct method and value format.

🚀 Application

expert

How many outputs does this Bitcoin transaction have after execution?

Given this simplified Bitcoin transaction creation code, how many outputs will the transaction have?

Blockchain / Solidity

const bitcoin = require('bitcoinjs-lib');
const keyPair = bitcoin.ECPair.makeRandom();
const { address } = bitcoin.payments.p2pkh({ pubkey: keyPair.publicKey });

const txb = new bitcoin.TransactionBuilder();
txb.addInput('some-txid', 0);
txb.addOutput('recipient-address', 50000);
txb.addOutput(address, 45000);

const tx = txb.build();
console.log(tx.outs.length);

A 2

B 1

C 3

D 0

💡 Hint

Count how many times addOutput is called.

The transaction builder adds two outputs: one to the recipient and one as change back to the sender's address. So, the transaction has 2 outputs.