Testing with ethers.js in Blockchain / Solidity - Mini Project: Build & Apply

Testing with ethers.js

📖 Scenario: You are building a simple smart contract that stores a number. You want to write tests using ethers.js to check if the contract works correctly.

🎯 Goal: Write a test script using ethers.js that deploys the contract, sets a number, and checks if the stored number is correct.

📋 What You'll Learn

Create a variable called SimpleStorage to get the contract factory

Create a variable called simpleStorage to deploy the contract

Create a variable called tx to store the transaction when setting the number

Use await tx.wait() to wait for the transaction to be mined

Use const storedNumber = await simpleStorage.retrieve() to get the stored number

Use console.log(storedNumber.toString()) to print the stored number

💡 Why This Matters

🌍 Real World

Testing smart contracts before deploying them to the blockchain helps avoid costly mistakes and bugs.

💼 Career

Blockchain developers use ethers.js and testing frameworks to ensure their contracts work correctly and securely.

1

Setup ethers.js and get the contract factory

Write const SimpleStorage = await ethers.getContractFactory('SimpleStorage') to get the contract factory for SimpleStorage.

Blockchain / Solidity

# Your code here

Hint

Use ethers.getContractFactory with the contract name 'SimpleStorage'.

2

Deploy the contract

Write const simpleStorage = await SimpleStorage.deploy() to deploy the contract and store it in simpleStorage.

Blockchain / Solidity

const SimpleStorage = await ethers.getContractFactory('SimpleStorage')
# Your code here

Hint

Call deploy() on SimpleStorage and assign it to simpleStorage.

3

Set a number in the contract

Write const tx = await simpleStorage.store(42) to call the store function with the number 42 and save the transaction in tx. Then write await tx.wait() to wait for the transaction to be mined.

Blockchain / Solidity

const SimpleStorage = await ethers.getContractFactory('SimpleStorage')
const simpleStorage = await SimpleStorage.deploy()
# Your code here

Hint

Call store(42) on simpleStorage and wait for the transaction to complete.

4

Retrieve and print the stored number

Write const storedNumber = await simpleStorage.retrieve() to get the stored number. Then write console.log(storedNumber.toString()) to print it.

Blockchain / Solidity

const SimpleStorage = await ethers.getContractFactory('SimpleStorage')
const simpleStorage = await SimpleStorage.deploy()
const tx = await simpleStorage.store(42)
await tx.wait()
# Your code here

Hint

Call retrieve() on simpleStorage and print the result as a string.

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