Writing test cases in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a simple test case function.

Blockchain / Solidity

function test[1]() {
  // test logic here
}

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

ATest

BCase

CRun

DCheck

Attempts:

3 left

💡 Hint

Common Mistakes

Using generic names like 'Run' or 'Check' instead of 'Test'.

The function name should start with test to indicate it is a test case.

2fill in blank

medium

Complete the code to assert that a smart contract function returns true.

Blockchain / Solidity

assert.equal(contract.[1](), true);

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

Arun

BisActive

Cexecute

Dtransfer

Attempts:

3 left

💡 Hint

Common Mistakes

Using functions that do not return booleans like 'transfer' or 'run'.

The function isActive is expected to return a boolean value, so asserting it equals true makes sense.

3fill in blank

hard

Fix the error in the test case to correctly check the balance after transfer.

Blockchain / Solidity

const balance = await contract.getBalance([1]);
assert.equal(balance.toNumber(), 100);

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

Aaccount

Breceiver

Caddress

Dsender

Attempts:

3 left

💡 Hint

Common Mistakes

Using generic terms like 'sender' or 'account' that may not be defined.

The getBalance function expects an address parameter to check the balance of that account.

4fill in blank

hard

Fill both blanks to write a test that checks if a transaction reverts with an error.

Blockchain / Solidity

await expect(contract.[1](100)).to.be.revertedWith([2]);

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Atransfer

Bapprove

C'Insufficient balance'

D'Unauthorized'

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong function names or error messages that don't match the test scenario.

The test checks that calling transfer with 100 tokens reverts with the error message 'Insufficient balance'.

5fill in blank

hard

Fill all three blanks to create a test that verifies an event is emitted with correct parameters.

Blockchain / Solidity

await expect(contract.[1](user, 50))
  .to.emit(contract, '[2]')
  .withArgs([3]);

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

Amint

BTransfer

Cuser, 50

DBurn

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong event names or incorrect event argument formats.

This test calls the mint function, expects the Transfer event, and checks that the event arguments are user and 50.

Practice

1. What is the main purpose of writing test cases for blockchain smart contracts?

easy

A. To find and fix errors before deploying the contract

B. To make the contract run faster on the blockchain

C. To increase the contract's gas cost

D. To make the contract code longer

Solution

1. Step 1: Understand the role of test cases

   Test cases are written to check if the code works correctly and to catch mistakes early.

2. Step 2: Relate to blockchain deployment

   Since blockchain contracts are hard to change after deployment, testing helps avoid costly errors.

3. Final Answer:

   To find and fix errors before deploying the contract -> Option A

4. Quick Check:

   Test cases catch errors early = C [OK]

Hint: Test cases catch errors before deployment [OK]

Common Mistakes:

• Thinking tests speed up contract execution
• Believing tests increase gas cost intentionally
• Assuming tests make code longer for no reason

2. Which of the following is the correct syntax to group tests in a blockchain testing framework like Mocha?

easy

A. itGroup('Token tests', () => { /* tests */ })

B. group('Token tests', () => { /* tests */ })

C. testGroup('Token tests', () => { /* tests */ })

D. describe('Token tests', () => { /* tests */ })

Solution

1. Step 1: Recall common test grouping syntax

   In Mocha and similar frameworks, describe is used to group related tests.

2. Step 2: Identify correct function name

   Only describe is the standard function for grouping; others are incorrect or do not exist.

3. Final Answer:

   describe('Token tests', () => { /* tests */ }) -> Option D

4. Quick Check:

   Group tests with describe() = A [OK]

Hint: Use describe() to group tests clearly [OK]

Common Mistakes:

• Using group() instead of describe()
• Confusing it() with describe()
• Inventing non-existent functions like testGroup()

3. Consider this test case snippet for a smart contract function that stores a value:

it('should store the value correctly', async () => {
  await contract.store(42);
  const result = await contract.retrieve();
  assert.equal(result.toNumber(), 42);
});

What will be the output if the contract stores 42 correctly?

medium

A. Test passes with no errors

B. Test fails with assertion error

C. Syntax error in test code

D. Runtime error due to missing function

Solution

1. Step 1: Analyze the test steps

   The test calls store(42) then retrieves the value and checks if it equals 42.

2. Step 2: Understand assertion behavior

   If the contract works correctly, result.toNumber() will be 42, so assert.equal passes without error.

3. Final Answer:

   Test passes with no errors -> Option A

4. Quick Check:

   Correct value stored means test passes = A [OK]

Hint: If assert matches expected, test passes [OK]

Common Mistakes:

• Assuming test fails even if values match
• Confusing syntax errors with assertion failures
• Ignoring async/await usage in tests

4. Identify the error in this test case for transferring tokens:

it('should transfer tokens', async () => {
  await contract.transfer(receiver, 100);
  const balance = await contract.balanceOf(receiver);
  assert.equal(balance, 100);
});

medium

A. Missing await before balanceOf call

B. No error, test is correct

C. Incorrect assertion comparing balance object to number

D. transfer function called without sender address

Solution

1. Step 1: Check async calls and returns

   The balanceOf likely returns a BigNumber object, not a plain number.

2. Step 2: Verify assertion correctness

   Comparing balance directly to 100 fails because types differ; conversion to number is needed.

3. Final Answer:

   Incorrect assertion comparing balance object to number -> Option C

4. Quick Check:

   Compare values with matching types = D [OK]

Hint: Convert BigNumber to number before assert [OK]

Common Mistakes:

• Forgetting to convert BigNumber to number
• Assuming balanceOf returns a plain number
• Missing await on async calls

5. You want to write a test case that checks if a smart contract correctly rejects unauthorized users from changing the owner. Which approach is best?

hard

A. Call the changeOwner function from the owner account and expect success

B. Call the changeOwner function from an unauthorized account and expect an error

C. Call the changeOwner function without any account specified

D. Skip testing unauthorized access because it is handled by blockchain

Solution

1. Step 1: Understand test goal

   The test should verify that unauthorized users cannot change ownership.

2. Step 2: Choose test action

   Calling changeOwner from a non-owner account and expecting an error confirms access control works.

3. Step 3: Eliminate wrong options

   Testing with owner account or no account does not check unauthorized rejection; skipping test misses bugs.

4. Final Answer:

   Call the changeOwner function from an unauthorized account and expect an error -> Option B

5. Quick Check:

   Test unauthorized access by expecting failure = B [OK]

Hint: Test unauthorized calls expect errors [OK]

Common Mistakes:

• Testing only authorized calls
• Ignoring unauthorized access tests
• Assuming blockchain prevents all unauthorized actions