Testing with ethers.js in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import ethers from the hardhat package.

Blockchain / Solidity

const { [1] } = require('hardhat');

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

Acontract

Bweb3

Cethers

Dprovider

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'web3' instead of 'ethers' in Hardhat tests.

We import ethers from the hardhat package to interact with Ethereum in tests.

2fill in blank

medium

Complete the code to deploy a contract named 'SimpleStorage'.

Blockchain / Solidity

const SimpleStorage = await ethers.getContractFactory([1]);
const simpleStorage = await SimpleStorage.deploy();

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

A'SimpleStorage'

B'Simple'

C'Storage'

D'StorageSimple'

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect or partial contract names.

The contract factory is created by passing the exact contract name as a string, here 'SimpleStorage'.

3fill in blank

hard

Fix the error in the test assertion to check if the stored value equals 42.

Blockchain / Solidity

expect(await simpleStorage.retrieve()).to.[1](42);

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

AtoBe

Bequals

Ceq

Dequal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'equals' or 'toBe' which are not valid Chai methods.

The correct Chai assertion method is equal to compare values.

4fill in blank

hard

Fill both blanks to wait for the deployment transaction to be mined.

Blockchain / Solidity

const tx = await simpleStorage.[1]();
await tx.[2]();

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

Astore

Bwait

Cdeploy

Dsend

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'deploy' or 'send' instead of 'store' for the function call.

We call the store function to send a transaction, then wait to wait for it to be mined.

5fill in blank

hard

Fill all three blanks to write a test that checks if the stored value updates correctly.

Blockchain / Solidity

it('updates stored value', async function() {
  const tx = await simpleStorage.[1](100);
  await tx.[2]();
  const result = await simpleStorage.[3]();
  expect(result).to.equal(100);
});

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

Astore

Bwait

Cretrieve

Dupdate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'update' instead of 'store' or 'retrieve'.

The test calls store to update, waits for mining with wait, then retrieves the value with retrieve to check it.

Practice

1. What is the main purpose of using ethers.js in testing smart contracts?

easy

A. To create new blockchain networks

B. To design the user interface of a blockchain app

C. To mine new blocks on the blockchain

D. To interact with smart contracts and verify their behavior programmatically

Solution

1. Step 1: Understand ethers.js role

   ethers.js is a library that helps developers interact with Ethereum smart contracts in code.

2. Step 2: Identify testing purpose

   In testing, ethers.js lets you call contract functions and check if they behave as expected.

3. Final Answer:

   To interact with smart contracts and verify their behavior programmatically -> Option D

4. Quick Check:

   ethers.js tests contract behavior = A [OK]

Hint: Remember: ethers.js tests contracts by calling their functions [OK]

Common Mistakes:

• Confusing ethers.js with UI design tools
• Thinking ethers.js mines blocks
• Assuming ethers.js creates blockchains

2. Which of the following is the correct way to get a signer from a provider in ethers.js?

easy

A. const signer = provider.signer();

B. const signer = provider.getSigner();

C. const signer = provider.getsigner();

D. const signer = provider.sign();

Solution

1. Step 1: Recall ethers.js signer method

   The correct method to get a signer from a provider is getSigner() with capital S.

2. Step 2: Check method spelling and casing

   Options B, C, and D have incorrect method names or casing.

3. Final Answer:

   const signer = provider.getSigner(); -> Option B

4. Quick Check:

   Correct method is getSigner() = A [OK]

Hint: Method names are case-sensitive; use getSigner() exactly [OK]

Common Mistakes:

• Using wrong casing like signer() instead of getSigner()
• Misspelling method names
• Confusing signer with sign

3. Consider this ethers.js test code snippet:

const tx = await contract.setValue(42);
await tx.wait();
const value = await contract.getValue();
console.log(value);

What will be printed to the console?

medium

A. 42

B. undefined

C. Transaction hash

D. Error: contract.getValue is not a function

Solution

1. Step 1: Understand transaction and wait

   The code calls setValue(42) on the contract, then waits for the transaction to confirm with tx.wait().

2. Step 2: Retrieve stored value

   After confirmation, it calls getValue() which returns the stored value 42.

3. Final Answer:

   42 -> Option A

4. Quick Check:

   Confirmed setValue(42) then getValue() = 42 [OK]

Hint: Always wait for tx confirmation before reading updated state [OK]

Common Mistakes:

• Not awaiting tx.wait() before reading value
• Expecting transaction hash instead of value
• Assuming getValue() is undefined

4. You wrote this test code but it throws an error:

const tx = contract.setValue(10);
await tx.wait();

What is the likely cause of the error?

medium

A. Missing await before contract.setValue(10)

B. tx.wait() is not a function

C. contract.setValue does not exist

D. Provider is not connected

Solution

1. Step 1: Check async call to contract method

   contract.setValue(10) returns a Promise, so it must be awaited to get the transaction object.

2. Step 2: Identify missing await issue

   Without await, tx is a Promise, which does not have a wait() method, causing the error.

3. Final Answer:

   Missing await before contract.setValue(10) -> Option A

4. Quick Check:

   Await async call before tx.wait() = C [OK]

Hint: Always await contract calls returning transactions before wait() [OK]

Common Mistakes:

• Forgetting await before async contract calls
• Assuming tx is transaction without awaiting
• Not checking provider connection first

5. You want to test a smart contract function transfer(address to, uint amount) that emits an event Transfer. Which ethers.js testing approach correctly waits for the transaction and verifies the event was emitted?

hard

A. await contract.transfer(addr, 100);
expect(tx.events[0].event).to.equal('Transfer');

B. const receipt = await contract.transfer(addr, 100);
expect(receipt.events[0].event).to.equal('Transfer');

C. const tx = await contract.transfer(addr, 100);
const receipt = await tx.wait();
expect(receipt.events[0].event).to.equal('Transfer');

D. const tx = contract.transfer(addr, 100);
expect(tx.events[0].event).to.equal('Transfer');

Solution

1. Step 1: Await the transaction call

   Use await to get the transaction object from contract.transfer.

2. Step 2: Wait for transaction confirmation and get receipt

   Call tx.wait() to get the receipt which contains event logs.

3. Step 3: Verify event in receipt

   Check receipt.events[0].event equals 'Transfer' to confirm event emitted.

4. Final Answer:

   const tx = await contract.transfer(addr, 100); const receipt = await tx.wait(); expect(receipt.events[0].event).to.equal('Transfer'); -> Option C

5. Quick Check:

   Await tx, wait receipt, check event = D [OK]

Hint: Always await tx, then wait for receipt to check events [OK]

Common Mistakes:

• Not awaiting transaction before wait()
• Trying to access events on tx instead of receipt
• Ignoring event verification after transaction