Writing test cases in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple smart contract test

Consider this Solidity test snippet using Hardhat framework. What will be the output printed by the test runner?

Blockchain / Solidity

describe('SimpleStorage', function () {
  it('should store and retrieve the value', async function () {
    const SimpleStorage = await ethers.getContractFactory('SimpleStorage');
    const storage = await SimpleStorage.deploy();
    await storage.deployed();

    await storage.store(42);
    const value = await storage.retrieve();
    console.log('Stored value:', value.toString());
  });
});

A Stored value: 42

B Stored value: 0

C TypeError: value.toString is not a function

D SyntaxError: Unexpected token

💡 Hint

Remember that retrieve() returns a BigNumber and toString() converts it to a string.

The contract stores 42, retrieve returns a BigNumber representing 42, and toString() converts it to '42'. So the console prints 'Stored value: 42'.

❓ Predict Output

intermediate

Test case result for failing assertion

What will happen when running this test case in a blockchain test framework?

Blockchain / Solidity

it('should fail when value is incorrect', async function () {
  const value = 10;
  assert.equal(value, 20, 'Value should be 20');
});

A TypeError: assert.equal is not a function

B Test passes silently

C SyntaxError: missing semicolon

D Test fails with message: Value should be 20

💡 Hint

assert.equal compares two values and throws if they differ.

The assertion compares 10 and 20, which are not equal, so the test fails with the given message.

🔧 Debug

advanced

Identify the error in this test case

This test case is supposed to check if a transaction reverts. What error will it produce when run?

Blockchain / Solidity

it('should revert on invalid input', async function () {
  await expect(contract.doSomething(0)).to.be.revertedWith('Invalid input');
});

A Test passes successfully

B TypeError: expect(...).to.be.revertedWith is not a function

C SyntaxError: Unexpected token await

D Error: contract.doSomething is not a function

💡 Hint

Check if the testing framework supports the 'revertedWith' matcher.

If the chai matchers for blockchain are not imported, 'revertedWith' is undefined causing a TypeError.

❓ Predict Output

advanced

Output of asynchronous test with event listening

What will this test print to the console when run?

Blockchain / Solidity

it('should emit event on action', async function () {
  const tx = await contract.performAction();
  const receipt = await tx.wait();
  const event = receipt.events.find(e => e.event === 'ActionPerformed');
  console.log('Event data:', event.args[0].toString());
});

A TypeError: receipt.events.find is not a function

B Event data: 123

C Event data: undefined

D SyntaxError: Unexpected identifier

💡 Hint

TransactionReceipt in ethers.js has 'logs' property, not 'events'.

In ethers.js, TransactionReceipt does not have an 'events' property (it has 'logs'). receipt.events is undefined, so receipt.events.find throws TypeError.

🚀 Application

expert

Number of test cases needed for full coverage

A smart contract function has 3 independent boolean inputs. How many test cases are needed to cover all input combinations?

A 3

B 6

C 8

D 9

💡 Hint

Each boolean can be true or false independently.

With 3 booleans, total combinations are 2^3 = 8.