Transaction confirmation handling in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this transaction confirmation check?

Consider this pseudocode for checking transaction confirmations on a blockchain. What will be the printed output?

Blockchain / Solidity

confirmations = 3
required_confirmations = 6
if confirmations >= required_confirmations:
    print("Transaction confirmed")
else:
    print(f"Waiting for {required_confirmations - confirmations} more confirmations")

A Waiting for 3 more confirmations

B Transaction confirmed

C Waiting for 6 more confirmations

D Error: variable not defined

💡 Hint

Check if the current confirmations meet or exceed the required confirmations.

The code compares current confirmations (3) with required (6). Since 3 < 6, it prints the waiting message with the difference (6-3=3).

🧠 Conceptual

intermediate

Which statement best describes transaction confirmation?

In blockchain, what does a transaction confirmation mean?

A The transaction has been included in a block and that block is part of the longest chain.

B The transaction is waiting in the mempool to be mined.

C The transaction has been rejected by the network.

D The transaction is being broadcast to peers.

💡 Hint

Think about when a transaction is considered secure and irreversible.

A confirmation means the transaction is inside a block that is accepted as part of the main blockchain, making it harder to reverse.

🔧 Debug

advanced

Why does this confirmation count code raise an error?

This code snippet is intended to count confirmations but raises an error. Identify the cause.

Blockchain / Solidity

def check_confirmations(tx_hash):
    confirmations = blockchain.get_confirmations(tx_hash)
    if confirmations > 0:
        return confirmations
    else:
        return 0

A IndentationError due to wrong indentation

B NameError because blockchain is not defined

C TypeError because confirmations is not an integer

D SyntaxError due to missing colon after if statement

💡 Hint

Check the syntax of the if statement line.

The if statement is missing a colon at the end, causing a SyntaxError.

🚀 Application

advanced

How many confirmations does this code wait for before proceeding?

Given this code snippet, how many confirmations does it require before printing 'Confirmed'?

Blockchain / Solidity

required_confirmations = 12
current_confirmations = 10
while current_confirmations < required_confirmations:
    print(f"Current: {current_confirmations}, waiting...")
    current_confirmations += 1
print("Confirmed")

A 11

B 12

C 2

D 10

💡 Hint

Look at the condition that stops the loop.

The loop runs until current_confirmations reaches 12, so it waits for 12 confirmations before printing 'Confirmed'.

❓ Predict Output

expert

What is the output of this asynchronous confirmation checker?

This JavaScript async function checks confirmations. What will it print?

Blockchain / Solidity

async function checkTx(txId) {
  let confirmations = 0;
  while (confirmations < 3) {
    confirmations++;
    console.log(`Confirmations: ${confirmations}`);
    await new Promise(resolve => setTimeout(resolve, 100));
  }
  console.log('Transaction confirmed');
}
checkTx('abc123');

A Confirmations: 0\nConfirmations: 1\nConfirmations: 2

B Transaction confirmed

C Confirmations: 1\nConfirmations: 2\nConfirmations: 3\nTransaction confirmed

D SyntaxError due to await outside async function

💡 Hint

The function increments confirmations and waits 100ms each loop until 3 confirmations.

The loop prints confirmations 1, 2, 3 with delays, then prints 'Transaction confirmed'.

Practice

1. What does transaction confirmation mean in blockchain?

easy

A. It means the transaction is deleted from the blockchain.

B. It means the transaction is reversed by the user.

C. It means the transaction is pending and not yet sent.

D. It means the transaction is safely recorded on the blockchain.

Solution

1. Step 1: Understand transaction confirmation meaning

   Transaction confirmation means the blockchain network has recorded the transaction securely.

2. Step 2: Compare options with definition

   Only It means the transaction is safely recorded on the blockchain. correctly states that confirmation means safe recording on the blockchain.

3. Final Answer:

   It means the transaction is safely recorded on the blockchain. -> Option D

4. Quick Check:

   Transaction confirmation = safe recording [OK]

Hint: Confirmation means transaction is securely recorded [OK]

Common Mistakes:

• Confusing confirmation with transaction pending state
• Thinking confirmation means deletion or reversal
• Assuming confirmation means user approval

2. Which of the following is the correct way to wait for a transaction confirmation in JavaScript using async/await?

easy

A. await transaction.confirm();

B. transaction.wait();

C. await transaction.wait();

D. transaction.confirm();

Solution

1. Step 1: Identify correct async syntax

   To wait for a promise in JavaScript, use await before the async function call.

2. Step 2: Match function name for confirmation

   The standard method to wait for transaction confirmation is wait(), not confirm().

3. Final Answer:

   await transaction.wait(); -> Option C

4. Quick Check:

   Use await with wait() to confirm transaction [OK]

Hint: Use await with wait() method to confirm transaction [OK]

Common Mistakes:

• Forgetting to use await causing unresolved promises
• Using wrong method name like confirm()
• Calling wait() without await leading to no pause

3. What will be the output of this JavaScript code snippet?

async function confirmTx(tx) {
  const receipt = await tx.wait();
  return receipt.confirmations;
}

const fakeTx = {
  wait: () => Promise.resolve({ confirmations: 3 })
};

confirmTx(fakeTx).then(console.log);

medium

A. 3

B. Promise {<pending>}

C. undefined

D. Error: wait is not a function

Solution

1. Step 1: Understand async function behavior

   The function confirmTx awaits tx.wait() which resolves to an object with confirmations: 3.

2. Step 2: Return and log confirmations

   The function returns receipt.confirmations which is 3, and then(console.log) prints 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Await wait() returns confirmations = 3 [OK]

Hint: Await returns resolved value, then log confirmations [OK]

Common Mistakes:

• Expecting Promise instead of resolved value
• Confusing property name confirmations
• Missing await causing Promise output

4. Identify the error in this code snippet for waiting transaction confirmation:

async function waitForConfirmation(tx) {
  const receipt = tx.wait();
  console.log(receipt.confirmations);
}

waitForConfirmation(transaction);

medium

A. Missing await before tx.wait()

B. Incorrect property name confirmations

C. Function should not be async

D. console.log should be outside the function

Solution

1. Step 1: Check async call usage

   The function calls tx.wait() which returns a Promise, but does not use await.

2. Step 2: Understand consequences of missing await

   Without await, receipt is a Promise object, so receipt.confirmations is undefined.

3. Final Answer:

   Missing await before tx.wait() -> Option A

4. Quick Check:

   Always await async calls to get resolved value [OK]

Hint: Always await async calls to get actual result [OK]

Common Mistakes:

• Not using await with async functions
• Assuming Promise has properties directly
• Misplacing console.log inside async function

5. You want to wait for at least 5 confirmations before proceeding with a transaction. Which code snippet correctly implements this logic?

hard

A. const receipt = await tx.wait(5); if(receipt.confirmations < 5) { proceed(); }

B. const receipt = await tx.wait(5); if(receipt.confirmations >= 5) { proceed(); }

C. const receipt = await tx.wait(3); if(receipt.confirmations >= 5) { proceed(); }

D. const receipt = await tx.wait(); if(receipt.confirmations == 5) { proceed(); }

Solution

1. Step 1: Use wait() with confirmation count

   Calling tx.wait(5) waits until at least 5 confirmations are reached.

2. Step 2: Check confirmations before proceeding

   Check if receipt.confirmations >= 5 to ensure safe confirmation before calling proceed().

3. Final Answer:

   const receipt = await tx.wait(5); if(receipt.confirmations >= 5) { proceed(); } -> Option B

4. Quick Check:

   wait(5) ensures 5 confirmations before proceed [OK]

Hint: Use wait(5) and check confirmations >= 5 before proceed [OK]

Common Mistakes:

• Waiting fewer confirmations than needed
• Checking for exact 5 instead of >= 5
• Proceeding when confirmations are less than required