Hardhat testing setup in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Hardhat test snippet?

Consider this simple Hardhat test for a smart contract that stores a number. What will the test output when run?

Blockchain / Solidity

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

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

A Stored value: 0

B Stored value: undefined

C Stored value: null

D TypeError: value.toString is not a function

💡 Hint

Think about the default value of a uint256 in Solidity.

The default value of an uninitialized uint256 in Solidity is 0. The retrieve() function returns this value, which is a BigNumber object. Calling toString() converts it to '0'.

🧠 Conceptual

intermediate

Which Hardhat testing setup step is missing here?

You want to write tests for your smart contract using Hardhat. You have installed Hardhat and created a test file. You wrote tests but they fail because ethers is undefined. What step did you miss?

A Importing <code>ethers</code> from Hardhat in the test file

B Installing <code>ethers</code> package separately

C Running <code>npx hardhat compile</code> before tests

D Adding <code>mocha</code> as a dev dependency

💡 Hint

Check how you access Hardhat's built-in libraries in tests.

Hardhat injects ethers into the test environment, but you must import it explicitly in your test file using const { ethers } = require('hardhat'); to use it.

🔧 Debug

advanced

Why does this Hardhat test fail with a timeout?

Look at this test code snippet. The test never finishes and times out. What is the cause?

Blockchain / Solidity

describe('Token', function () {
  it('should mint tokens', async function () {
    const Token = await ethers.getContractFactory('Token');
    const token = await Token.deploy();
    // Missing await here
    await token.deployed();

    const tx = await token.mint(accounts[0].address, 100);
    await tx.wait();

    const balance = await token.balanceOf(accounts[0].address);
    console.log('Balance:', balance.toString());
  });
});

A mint function does not exist on the contract

B tx.wait() is called incorrectly without await

C accounts is not defined, causing a ReferenceError

D Missing await on token.deployed() causes the test to hang

💡 Hint

Check asynchronous calls that might block the test.

The token.deployed() call returns a promise that must be awaited. Without await, the test continues before deployment finishes, causing unpredictable behavior and timeout.

📝 Syntax

advanced

Which option fixes the syntax error in this Hardhat test snippet?

Identify the option that corrects the syntax error in this test code:

describe('MyContract', function() {
  it('does something', async function() {
    const contract = await ethers.getContractFactory('MyContract')
    const instance = await contract.deploy()
    await instance.deployed()

    const result = await instance.myFunction()
    console.log('Result:', result)
  )
})

A Change <code>async function()</code> to <code>async () =></code>

B Add a semicolon after <code>await instance.deployed()</code>

C Replace the closing parenthesis <code>)</code> after <code>console.log</code> with a closing curly brace <code>}</code>

D Add a missing comma after <code>describe</code> function

💡 Hint

Look carefully at the function closing syntax.

The it callback function is closed with a parenthesis ) instead of a curly brace }. This causes a syntax error. Replacing it with } fixes the block closure.

🚀 Application

expert

How many tests run with this Hardhat test file?

Given this test file, how many individual tests will run when executing npx hardhat test?

Blockchain / Solidity

describe('ContractA', function () {
  it('test 1', async function () {});
  it('test 2', async function () {});

  describe('Nested Suite', function () {
    it('test 3', async function () {});
  });
});

describe('ContractB', function () {
  it('test 4', async function () {});
});

A 3

B 4

C 2

D 5

💡 Hint

Count all it blocks, including nested ones.

There are 4 it blocks total: two in ContractA top level, one nested inside Nested Suite, and one in ContractB.

Practice

1. What is the main purpose of using Hardhat testing setup in blockchain development?

easy

A. To create user interfaces for blockchain applications

B. To deploy smart contracts directly to the main Ethereum network

C. To write and run tests for smart contracts on a local blockchain

D. To mine real cryptocurrency coins

Solution

1. Step 1: Understand Hardhat testing role

   Hardhat testing setup is designed to help developers write and run tests for smart contracts safely.

2. Step 2: Identify the environment used

   Tests run on a local blockchain, which means no real money or main network is involved.

3. Final Answer:

   To write and run tests for smart contracts on a local blockchain -> Option C

4. Quick Check:

   Hardhat testing = local smart contract tests [OK]

Hint: Hardhat tests run locally, not on mainnet [OK]

Common Mistakes:

• Confusing testing with deployment to mainnet
• Thinking Hardhat mines real coins
• Assuming Hardhat builds user interfaces

2. Which of the following is the correct way to start a test suite in Hardhat using Mocha?

easy

A. test('MyContract', function() { ... })

B. describe('MyContract', function() { ... })

C. suite('MyContract', () => { ... })

D. context('MyContract', function() { ... })

Solution

1. Step 1: Recall Mocha test structure

   Hardhat uses Mocha for testing, which organizes tests with describe blocks.

2. Step 2: Identify correct syntax

   The describe function takes a string and a callback function to group tests.

3. Final Answer:

   describe('MyContract', function() { ... }) -> Option B

4. Quick Check:

   Mocha test suites use describe() [OK]

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

Common Mistakes:

• Using test() instead of describe() for suites
• Confusing suite() or context() with Mocha syntax
• Missing function keyword or arrow function

3. Given this Hardhat test snippet, what will be the output when running the test?

describe('SimpleTest', function() {
  it('should return true', async function() {
    const result = true;
    console.log(result);
    expect(result).to.equal(true);
  });
});

medium

A. true and test passes

B. false and test fails

C. undefined and test passes

D. Error: expect is not defined

Solution

1. Step 1: Analyze console output

   The code logs the variable result which is set to true, so console shows 'true'.

2. Step 2: Check assertion

   The test asserts result equals true, which is correct, so test passes.

3. Final Answer:

   true and test passes -> Option A

4. Quick Check:

   console.log(true) + expect(true) passes [OK]

Hint: Check console.log and expect values carefully [OK]

Common Mistakes:

• Assuming expect is undefined without import
• Confusing console output with test result
• Thinking test fails because of async function

4. Identify the error in this Hardhat test code snippet:

describe('Token', function() {
  it('should deploy contract', async () => {
    this.timeout(20000);
    const Token = await ethers.getContractFactory('Token');
    const token = await Token.deploy();
    await token.waitForDeployment();
    expect(token.address).to.be.a('string');
  });
});

medium

A. Missing import of ethers from Hardhat

B. deploy() should not be awaited

C. Incorrect assertion method for address type

D. Using arrow function in it() causes context loss

Solution

1. Step 1: Check function style in it()

   Using arrow functions in Mocha's it blocks can cause loss of this context, which Hardhat sometimes relies on.

2. Step 2: Verify other parts

   Imports and await usage are correct; assertion syntax is valid for checking string type.

3. Final Answer:

   Using arrow function in it() causes context loss -> Option D

4. Quick Check:

   Use function() not arrow in it() [OK]

Hint: Avoid arrow functions in it() blocks [OK]

Common Mistakes:

• Ignoring context loss with arrow functions
• Thinking deploy() is synchronous
• Assuming ethers import is automatic

5. You want to test a smart contract function that transfers tokens and emits an event. Which setup correctly tests the event emission using Hardhat and Chai?

hard

A. await expect(token.transfer(addr1.address, 100)).to.emit(token, 'Transfer').withArgs(owner.address, addr1.address, 100);

B. expect(token.transfer(addr1.address, 100)).to.emit('Transfer').withArgs(owner.address, addr1.address, 100);

C. await token.transfer(addr1.address, 100); expect(token).to.emit('Transfer');

D. token.transfer(addr1.address, 100); expect(token).to.emit(token, 'Transfer');

Solution

1. Step 1: Understand event testing syntax

   Hardhat with Chai uses await expect(transaction).to.emit(contract, eventName).withArgs(...) to test events.

2. Step 2: Check options for correct usage

   await expect(token.transfer(addr1.address, 100)).to.emit(token, 'Transfer').withArgs(owner.address, addr1.address, 100); correctly awaits the transaction and specifies contract and event with arguments.

3. Step 3: Identify errors in other options

   The other options either miss the await before expect, fail to specify the contract instance in to.emit(), or use incorrect syntax chaining.

4. Final Answer:

   await expect(token.transfer(addr1.address, 100)).to.emit(token, 'Transfer').withArgs(owner.address, addr1.address, 100); -> Option A

5. Quick Check:

   Use await expect(...).to.emit(contract, event) [OK]

Hint: Always await expect() when testing events [OK]

Common Mistakes:

• Forgetting to await the expect call
• Not specifying the contract in to.emit()
• Calling expect on transaction result instead of promise