Testing with ethers.js in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple ethers.js contract call

What is the output of this ethers.js code snippet when calling a contract's getValue function that returns 42?

Blockchain / Solidity

const { ethers } = require('ethers');

async function test() {
  const provider = new ethers.providers.JsonRpcProvider();
  const contractAddress = '0x1234567890123456789012345678901234567890';
  const abi = [
    'function getValue() view returns (uint256)'
  ];
  const contract = new ethers.Contract(contractAddress, abi, provider);
  const value = await contract.getValue();
  console.log(value.toString());
}
test();

A 42

B undefined

C Error: contract method not found

D 0

💡 Hint

Remember that ethers.js returns BigNumber objects for uint256 values, so you need to convert them to string or number.

The contract's getValue returns 42 as a BigNumber. Calling toString() converts it to the string '42'.

❓ Predict Output

intermediate

Result of sending a transaction with ethers.js signer

What will be the value of txResponse.hash after sending this transaction?

Blockchain / Solidity

const { ethers } = require('ethers');

async function sendTx() {
  const provider = new ethers.providers.JsonRpcProvider();
  const signer = provider.getSigner();
  const txResponse = await signer.sendTransaction({
    to: '0xabcdefabcdefabcdefabcdefabcdefabcdefabcd',
    value: ethers.utils.parseEther('0.01')
  });
  console.log(txResponse.hash);
}
sendTx();

A A 66-character hexadecimal string starting with '0x'

B The number 0.01

C undefined

D Error: signer not connected

💡 Hint

Transaction hashes are always 32 bytes represented as 66 hex characters including '0x'.

The txResponse.hash is the transaction hash, a 32-byte hex string starting with '0x' and 64 hex digits after.

🔧 Debug

advanced

Identify the error in this ethers.js test setup

What error will this ethers.js test code produce when run?

Blockchain / Solidity

const { ethers } = require('ethers');

describe('MyContract', () => {
  let contract;
  beforeEach(async () => {
    const factory = await ethers.getContractFactory('MyContract');
    contract = await factory.deploy();
    await contract.deployed();
  });

  it('should return correct value', async () => {
    const value = await contract.getValue();
    expect(value).to.equal(42);
  });
});

A No error, test passes

B TypeError: ethers.getContractFactory is not a function

C SyntaxError: Unexpected token 'await'

D ReferenceError: expect is not defined

💡 Hint

Check if the testing assertion library is imported or available.

The code uses expect without importing it from a test library like Chai, causing a ReferenceError.

📝 Syntax

advanced

Find the syntax error in this ethers.js async function

Which option correctly fixes the syntax error in this code?

Blockchain / Solidity

async function test() {
  const provider = new ethers.providers.JsonRpcProvider();
  const balance = provider.getBalance('0xabc123...');
  console.log(balance.toString());
}
test();

A Change function to non-async: function test() { ... }

B Add await before provider.getBalance: const balance = await provider.getBalance('0xabc123...');

C Add .then() to getBalance: provider.getBalance('0xabc123...').then(balance => console.log(balance.toString()));

D No fix needed, code runs fine

💡 Hint

Remember that getBalance returns a Promise and must be awaited inside async functions.

Without await, balance is a Promise, so calling toString() on it causes an error. Adding await fixes this.

🚀 Application

expert

How to test event emission with ethers.js and Mocha

You want to test that your contract emits an event ValueChanged with the new value 100 after calling setValue(100). Which code snippet correctly tests this using ethers.js and Mocha?

A

contract.setValue(100).then(() => {
  console.log('Event emitted');
});

B

const tx = await contract.setValue(100);
await tx.wait();
const events = await contract.queryFilter('ValueChanged');
assert(events[0].args.value === 100);

C await expect(contract.setValue(100)).to.emit(contract, 'ValueChanged').withArgs(100);

D

const event = contract.filters.ValueChanged(100);
expect(event).to.exist;

💡 Hint

Use the Chai matchers from ethereum-waffle or hardhat for event testing.

Option C uses the correct syntax with expect and to.emit matcher to test event emission with arguments.