Sending transactions in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to send a transaction using the web3 library.

Blockchain / Solidity

const tx = {
  from: senderAddress,
  to: receiverAddress,
  value: web3.utils.toWei('1', '[1]')
};

Drag options to blanks, or click blank then click option'

Aether

Bwei

Cgwei

Dtoken

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'wei' directly without conversion.

Using 'gwei' which is a smaller unit.

Using an invalid unit like 'token'.

The value must be converted to wei from ether using web3.utils.toWei with 'ether' as the unit.

2fill in blank

medium

Complete the code to sign the transaction with the private key.

Blockchain / Solidity

const signedTx = await web3.eth.accounts.signTransaction(tx, [1]);

Drag options to blanks, or click blank then click option'

AtransactionHash

BreceiverAddress

CsenderAddress

DprivateKey

Attempts:

3 left

💡 Hint

Common Mistakes

Using the sender's address instead of the private key.

Using the receiver's address.

Using the transaction hash which is not available yet.

The transaction must be signed with the sender's private key to authorize it.

3fill in blank

hard

Fix the error in sending the signed transaction.

Blockchain / Solidity

web3.eth.sendSignedTransaction([1].rawTransaction)
  .on('receipt', console.log);

Drag options to blanks, or click blank then click option'

AsignedTx

Btx

Ctransaction

Dreceipt

Attempts:

3 left

💡 Hint

Common Mistakes

Using the original transaction object instead of the signed one.

Using undefined variables.

Using the receipt object before sending.

The signed transaction object is signedTx, which contains the rawTransaction property.

4fill in blank

hard

Fill both blanks to create a transaction object with gas and gasPrice.

Blockchain / Solidity

const tx = {
  from: senderAddress,
  to: receiverAddress,
  value: web3.utils.toWei('0.5', 'ether'),
  gas: [1],
  gasPrice: web3.utils.toWei('[2]', 'gwei')
};

Drag options to blanks, or click blank then click option'

A21000

B50

C100000

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Using too low gas limit causing transaction failure.

Using gas price in wei instead of gwei.

Confusing gas and gasPrice values.

The standard gas limit for a simple ETH transfer is 21000, and a common gas price is 50 gwei.

5fill in blank

hard

Fill all three blanks to create a transaction receipt handler.

Blockchain / Solidity

web3.eth.sendSignedTransaction(signedTx.rawTransaction)
  .on('[1]', (receipt) => {
    console.log('Transaction [2] with hash:', receipt.[3]);
  });

Drag options to blanks, or click blank then click option'

Areceipt

Bconfirmation

CtransactionHash

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'error' event instead of 'receipt'.

Logging wrong receipt properties.

Confusing event names.

The event to listen for is 'receipt', the message says 'confirmation', and the receipt object has the 'transactionHash' property.

Practice

1.

What is the main purpose of sending a transaction on a blockchain?

easy

A. To create a new blockchain network

B. To mine new blocks

C. To move value or data from one account to another

D. To delete data from the blockchain

Solution

1. Step 1: Understand what a transaction does

   A transaction moves value or data between accounts on the blockchain.

2. Step 2: Compare options to the definition

   Only To move value or data from one account to another correctly describes sending a transaction.

3. Final Answer:

   To move value or data from one account to another -> Option C

4. Quick Check:

   Transaction purpose = move value/data [OK]

Hint: Transactions move value or data, not create or delete [OK]

Common Mistakes:

• Confusing transactions with mining
• Thinking transactions create blockchains
• Believing transactions delete blockchain data

2.

Which of the following is the correct way to sign a transaction before sending it?

transaction.sign(____)

easy

A. sender's public key

B. sender's private key

C. receiver's private key

D. network's public key

Solution

1. Step 1: Identify the key needed for signing

   Transactions must be signed with the sender's private key to prove ownership.

2. Step 2: Match the correct key to the method

   Only the sender's private key can sign the transaction securely.

3. Final Answer:

   sender's private key -> Option B

4. Quick Check:

   Sign with private key = sender's private key [OK]

Hint: Sign with sender's private key, never public key [OK]

Common Mistakes:

• Using public key to sign
• Using receiver's key instead of sender's
• Confusing private and public keys

3.

Consider this code snippet sending a transaction:

tx = {
  'to': '0xabc123',
  'value': 10,
  'nonce': 5,
  'gas': 21000
}
signed_tx = sign_transaction(tx, private_key)
result = send_transaction(signed_tx)
print(result)

What will print(result) most likely output if the nonce is incorrect?

medium

A. 'Nonce too low or too high error'

B. 'Transaction successful'

C. 'Gas limit exceeded error'

D. 'Invalid signature error'

Solution

1. Step 1: Understand nonce role in transactions

   Nonce must be correct and sequential to avoid replay or duplication errors.

2. Step 2: Identify error caused by wrong nonce

   An incorrect nonce causes a 'Nonce too low or too high' error during sending.

3. Final Answer:

   'Nonce too low or too high error' -> Option A

4. Quick Check:

   Wrong nonce = nonce error [OK]

Hint: Wrong nonce causes nonce error, not gas or signature errors [OK]

Common Mistakes:

• Confusing nonce error with gas error
• Assuming signature error for nonce issues
• Expecting success despite wrong nonce

4.

Given this code snippet, what is the main error preventing the transaction from sending?

tx = {
  'to': '0xdef456',
  'value': 5,
  'nonce': 3
}
signed_tx = sign_transaction(tx, private_key)
result = send_transaction(signed_tx)
print(result)

Options:

medium

A. Receiver address is invalid

B. Incorrect private key used for signing

C. Nonce value is too high

D. Missing gas field in the transaction

Solution

1. Step 1: Check required transaction fields

   Gas is required to pay for transaction processing; missing gas causes failure.

2. Step 2: Verify other fields and keys

   Nonce and receiver address look valid; no info about wrong private key.

3. Final Answer:

   Missing gas field in the transaction -> Option D

4. Quick Check:

   Missing gas = transaction fails [OK]

Hint: Always include gas field to avoid transaction failure [OK]

Common Mistakes:

• Ignoring gas field requirement
• Assuming nonce or address is wrong without evidence
• Not checking transaction structure

5.

You want to send multiple transactions quickly from the same account. Which approach ensures all transactions are accepted without nonce conflicts?

1. Use the same nonce for all transactions
2. Increment nonce by 1 for each transaction
3. Use random nonce values
4. Skip nonce and rely on network

hard

A. Increment nonce by 1 for each transaction

B. Skip nonce and rely on network

C. Use random nonce values

D. Use the same nonce for all transactions

Solution

1. Step 1: Understand nonce role in multiple transactions

   Nonce must be unique and sequential per account to avoid conflicts.

2. Step 2: Identify correct nonce handling method

   Incrementing nonce by 1 for each transaction ensures proper ordering and acceptance.

3. Final Answer:

   Increment nonce by 1 for each transaction -> Option A

4. Quick Check:

   Sequential nonce = no conflicts [OK]

Hint: Always increment nonce by 1 for each new transaction [OK]

Common Mistakes:

• Reusing same nonce causing rejection
• Using random nonce causing errors
• Assuming network assigns nonce automatically