Writing test cases in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Writing test cases

Identify feature to test

↓

Write test case: inputs, expected output

↓

Run test case

↓

Compare actual vs expected

↓

Confirm code

↓

Repeat

This flow shows how to write and run test cases to check if blockchain code works as expected.

Execution Sample

Blockchain / Solidity

function testTransfer() {
  let balanceBefore = getBalance(user);
  transfer(user, 10);
  let balanceAfter = getBalance(user);
  assert(balanceAfter === balanceBefore - 10);
}

A simple test case checking if transferring 10 tokens reduces user's balance by 10.

Execution Table

Step	Action	Value/Check	Result
1	Get balanceBefore	balanceBefore = 100	Stored 100
2	Call transfer(user, 10)	Balance reduces by 10	Transfer done
3	Get balanceAfter	balanceAfter = 90	Stored 90
4	Assert balanceAfter === balanceBefore - 10	90 === 100 - 10	True, test passes

💡 Test ends after assertion passes confirming transfer works correctly

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
balanceBefore	undefined	100	100	100
balanceAfter	undefined	undefined	90	90

Key Moments - 2 Insights

Why do we check balanceBefore before transfer?

We need the original balance to compare after transfer. See execution_table step 1 and 3 where balanceBefore and balanceAfter are stored.

What if the assertion fails?

If assertion fails (step 4), it means transfer did not reduce balance correctly. We must fix code or test before continuing.

Visual Quiz - 3 Questions

Look at the execution table, what is balanceAfter at step 3?

A 10

B 90

C 100

D Undefined

💡 Hint

Check the 'Value/Check' column at step 3 in execution_table

At which step do we confirm the test passes?

A Step 1

B Step 2

C Step 4

D Step 3

💡 Hint

Look at the 'Result' column for the step where assertion is checked

If transfer did not reduce balance, which step would fail?

A Step 4

B Step 1

C Step 2

D Step 3

💡 Hint

Assertion check in step 4 compares expected and actual balances

Concept Snapshot

Writing test cases:
1. Identify feature to test.
2. Write test with inputs and expected output.
3. Run test and capture actual output.
4. Compare actual vs expected.
5. Pass means code works; fail means fix needed.
Repeat for confidence.

Full Transcript

Writing test cases in blockchain means checking if code works as expected. First, pick what to test. Then write a test case with inputs and what you expect to happen. Run the test and get actual results. Compare actual results with expected. If they match, test passes. If not, fix code or test. Repeat this process to make sure your blockchain code is correct and safe.

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