Testing with ethers.js in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is ethers.js used for in blockchain development?

ethers.js is a library that helps developers interact with the Ethereum blockchain. It allows you to write scripts and tests to communicate with smart contracts, send transactions, and read blockchain data.

beginner

How do you create a provider in ethers.js for testing?

<p>You create a provider using <code>ethers.providers.JsonRpcProvider</code> or use the default provider for local testing networks like Hardhat or Ganache.</p><p>Example:<br><code>const provider = new ethers.providers.JsonRpcProvider();</code></p>

beginner

What is the purpose of a signer in ethers.js tests?

A signer represents an Ethereum account that can sign transactions. In tests, signers are used to simulate different users sending transactions or interacting with contracts.

intermediate

How do you deploy a smart contract in a test using ethers.js?

<p>First, get the contract factory with <code>ethers.getContractFactory</code>. Then call <code>deploy()</code> on it and wait for deployment.</p><p>Example:<br><code>const MyContract = await ethers.getContractFactory('MyContract');<br>const contract = await MyContract.deploy();<br>await contract.deployed();</code></p>

beginner

Why is it important to use await when calling contract functions in tests?

Because blockchain interactions are asynchronous, await ensures the test waits for the transaction or call to complete before moving on. This prevents race conditions and incorrect test results.

Which ethers.js object represents an Ethereum account that can send transactions?

A Signer

B ContractFactory

C Provider

D Transaction

A Signer represents an Ethereum account that can sign and send transactions.

What method do you use to get a contract factory in ethers.js for deployment?

A ethers.getContract()

B ethers.getContractFactory()

C ethers.deployContract()

D ethers.createContract()

ethers.getContractFactory() returns a factory to deploy new contract instances.

Why do you use await when calling contract functions in tests?

A To speed up the test

B To run multiple tests at once

C To wait for the transaction to complete

D To ignore errors

await waits for the asynchronous blockchain call or transaction to finish before continuing.

Which provider is commonly used for local blockchain testing with ethers.js?

A InfuraProvider

B WebSocketProvider

C AlchemyProvider

D JsonRpcProvider

JsonRpcProvider connects to local blockchain nodes like Hardhat or Ganache for testing.

What does contract.deployed() do in ethers.js tests?

A Waits until the contract is fully deployed

B Returns the contract address

C Deploys the contract

D Compiles the contract

contract.deployed() waits for the deployment transaction to be mined and confirmed.

Explain the steps to write a simple test using ethers.js to deploy a contract and call one of its functions.

Think about how you prepare, deploy, and interact with the contract in a test.

Describe the role of providers and signers in ethers.js testing environment.

Consider who talks to the blockchain and who acts as the user.

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