Event testing in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to emit an event named Transfer.

Blockchain / Solidity

emit [1](msg.sender, recipient, amount);

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

ADeposit

BApproval

CTransfer

DWithdraw

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong event name like Approval instead of Transfer.

Forgetting to emit the event.

The Transfer event is emitted to signal token transfers.

2fill in blank

medium

Complete the code to listen for the Approval event in a test.

Blockchain / Solidity

const filter = contract.filters.[1](owner, spender);

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

AWithdraw

BApproval

CDeposit

DTransfer

Attempts:

3 left

💡 Hint

Common Mistakes

Using Transfer event filter when testing Approval.

Misspelling the event name.

The Approval event is used to track allowance changes.

3fill in blank

hard

Fix the error in the test assertion for event arguments.

Blockchain / Solidity

expect(event.args.[1]).to.equal(recipient);

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

Asender

Bowner

Camount

Drecipient

Attempts:

3 left

💡 Hint

Common Mistakes

Using sender instead of recipient for the receiver address.

Confusing argument names.

The recipient argument holds the address receiving tokens.

4fill in blank

hard

Fill both blanks to filter events by sender and minimum amount.

Blockchain / Solidity

const filter = contract.filters.Transfer([1], null, [2]);

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

AsenderAddress

BrecipientAddress

CminAmount

DmaxAmount

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping sender and recipient addresses.

Using maxAmount instead of minAmount.

The filter uses senderAddress to specify who sent tokens and minAmount to filter transfers above a value.

5fill in blank

hard

Fill all three blanks to assert event emission with correct args.

Blockchain / Solidity

await expect(tx).to.emit(contract, '[1]').withArgs([2], [3], amount);

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

ATransfer

BsenderAddress

CrecipientAddress

DApproval

Attempts:

3 left

💡 Hint

Common Mistakes

Using Approval event instead of Transfer.

Swapping sender and recipient arguments.

The test expects the Transfer event with senderAddress and recipientAddress as first two arguments.

Practice

1.

What is the main purpose of event testing in blockchain smart contracts?

easy

A. To verify that the contract emits the correct events during execution

B. To check the contract's gas usage

C. To test the contract's user interface

D. To optimize the contract's storage

Solution

1. Step 1: Understand event testing purpose

   Event testing focuses on confirming that smart contracts emit the right signals (events) when certain actions happen.

2. Step 2: Compare options with event testing goal

   Only To verify that the contract emits the correct events during execution matches this purpose, as it talks about verifying emitted events.

3. Final Answer:

   To verify that the contract emits the correct events during execution -> Option A

4. Quick Check:

   Event testing = Verify emitted events [OK]

Hint: Event testing checks emitted signals, not gas or UI [OK]

Common Mistakes:

• Confusing event testing with gas optimization
• Thinking event testing checks UI elements
• Assuming event testing measures storage use

2.

Which of the following is the correct syntax to listen for an event named Transfer in a Solidity test using JavaScript?

contractInstance.Transfer({fromBlock: 0, toBlock: 'latest'}).???

easy

A. on

B. watch

C. emit

D. listen

Solution

1. Step 1: Recall event listening syntax in Solidity tests

   In older Solidity test frameworks, watch is used to listen for events on contract instances.

2. Step 2: Match options with correct method

   Only watch fits the syntax shown; on, emit, and listen are incorrect here.

3. Final Answer:

   watch -> Option B

4. Quick Check:

   Event listening method = watch [OK]

Hint: Use 'watch' to listen for events in Solidity JS tests [OK]

Common Mistakes:

• Using 'emit' which triggers events, not listens
• Confusing 'on' with event listening in other frameworks
• Using 'listen' which is not a Solidity test method

3.

Given this Solidity event and test snippet, what will be the output of console.log(events.length) after calling transfer() once?

event Transfer(address indexed from, address indexed to, uint amount);

// In test:
const events = await contractInstance.getPastEvents('Transfer', {fromBlock: 0, toBlock: 'latest'});
await contractInstance.transfer(receiver, 100);
console.log(events.length);

medium

A. undefined

B. 1

C. 0

D. 100

Solution

1. Step 1: Understand event fetching timing

   The getPastEvents call happens before transfer(), so it fetches events before the transfer.

2. Step 2: Analyze event count after transfer

   Since events are fetched before the transfer, events.length will be 0 because no Transfer event happened yet.

3. Final Answer:

   0 -> Option C

4. Quick Check:

   Events fetched before transfer = 0 [OK]

Hint: Fetch events after action to see new events [OK]

Common Mistakes:

• Assuming events array updates automatically
• Ignoring async order of calls
• Expecting event count before event occurs

4.

Identify the error in this test code snippet that listens for an event Approval:

contractInstance.Approval().watch((error, event) => {
  if (!error) {
    console.log('Approval event:', event);
  }
});

medium

A. Using watch instead of on

B. Using Approval() instead of Approval without parentheses

C. Incorrect callback function signature

D. Missing event filter parameters in the call

Solution

1. Step 1: Check event listening syntax

   The event constructor requires a filter object parameter (e.g., {fromBlock: 0}) to specify the event scope.

2. Step 2: Identify correct usage

   The correct syntax is contractInstance.Approval({fromBlock: 0}).watch(...); the code uses empty Approval().

3. Final Answer:

   Missing event filter parameters in the call -> Option D

4. Quick Check:

   Event filter params required [OK]

Hint: Include event filter parameters like {fromBlock: 0} [OK]

Common Mistakes:

• Omitting filter parameters in event call
• Confusing callback parameters
• Mixing event listening methods

5.

You want to test that a Deposit event is emitted only when the deposit amount is greater than zero. Which test approach correctly verifies this?

1. Call deposit(0) and check no event emitted
2. Call deposit(100) and check event emitted with amount 100
3. Call deposit(-50) and check event emitted
4. Call deposit(100) and check event emitted with amount 0

hard

A. Only steps 1 and 2 are correct test cases

B. All steps are correct to test event emission

C. Only step 3 is needed to test event emission

D. Only step 4 is correct to test event emission

Solution

1. Step 1: Understand event emission condition

   The event should emit only if deposit amount > 0, so deposit(0) should emit no event, deposit(100) should emit event with amount 100.

2. Step 2: Evaluate test steps

   Step 3 calls deposit(-50), which is invalid (negative deposit), so event emission is not expected. Step 4 expects event with amount 0, which contradicts the condition.

3. Final Answer:

   Only steps 1 and 2 are correct test cases -> Option A

4. Quick Check:

   Test event only for positive deposits [OK]

Hint: Test zero and positive amounts only for event emission [OK]

Common Mistakes:

• Testing negative amounts which are invalid
• Expecting event with zero amount
• Assuming all deposits emit events