Web3.js vs ethers.js in Blockchain / Solidity - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Web3.js contract call

What will be the output of this Web3.js code snippet when calling a contract method that returns a string?

Blockchain / Solidity

const Web3 = require('web3');
const web3 = new Web3('https://mainnet.infura.io/v3/YOUR_PROJECT_ID');
const contract = new web3.eth.Contract(abi, contractAddress);

async function getName() {
  const name = await contract.methods.name().call();
  console.log(name);
}

getName();

A "MyToken"

B undefined

C TypeError: contract.methods.name is not a function

D Promise { <pending> }

💡 Hint

Remember that .call() returns the actual value asynchronously.

The .call() method in Web3.js returns a Promise that resolves to the value returned by the contract method. Here, it prints the string returned by the name() method.

❓ Predict Output

intermediate

Output of ethers.js contract call

What will this ethers.js code print when calling a contract method that returns a number?

Blockchain / Solidity

import { ethers } from 'ethers';

const provider = new ethers.JsonRpcProvider('https://mainnet.infura.io/v3/YOUR_PROJECT_ID');
const contract = new ethers.Contract(contractAddress, abi, provider);

async function getTotalSupply() {
  const supply = await contract.totalSupply();
  console.log(supply.toString());
}

getTotalSupply();

A BigNumber { _hex: '0x0f4240', _isBigNumber: true }

B undefined

C 1000000

D TypeError: contract.totalSupply is not a function

💡 Hint

ethers.js returns BigNumber objects for big integers; use toString() to print.

The totalSupply() method returns a BigNumber. Calling toString() converts it to a readable string number.

🔧 Debug

advanced

Identify the error in Web3.js event subscription

What error will this Web3.js code produce when trying to subscribe to a contract event?

Blockchain / Solidity

const Web3 = require('web3');
const web3 = new Web3('wss://mainnet.infura.io/ws/v3/YOUR_PROJECT_ID');
const contract = new web3.eth.Contract(abi, contractAddress);

contract.events.Transfer((error, event) => {
  if (error) console.error(error);
  else console.log(event);
});

A TypeError: contract.events.Transfer is not a function

B No error, logs Transfer events correctly

C Error: Invalid JSON RPC response

D Error: WebSocket connection failed

💡 Hint

Check if the WebSocket URL and event subscription syntax are correct.

The code correctly subscribes to the Transfer event using Web3.js with a valid WebSocket provider, so it logs events without error.

❓ Predict Output

advanced

ethers.js signer transaction output

What will this ethers.js code print after sending a transaction?

Blockchain / Solidity

import { ethers } from 'ethers';

const provider = new ethers.JsonRpcProvider('https://mainnet.infura.io/v3/YOUR_PROJECT_ID');
const signer = new ethers.Wallet(privateKey, provider);
const contract = new ethers.Contract(contractAddress, abi, signer);

async function sendTx() {
  const txResponse = await contract.transfer('0xRecipientAddress', 1000);
  console.log(txResponse.hash);
}

sendTx();

A TypeError: contract.transfer is not a function

B undefined

C Error: signer.transfer is not a function

D 0x123abc... (transaction hash string)

💡 Hint

Transaction methods return a transaction response object with a hash property.

The transfer method sends a transaction and returns a transaction response. The hash property is the transaction hash string.

🧠 Conceptual

expert

Key difference between Web3.js and ethers.js providers

Which statement best describes a key difference between Web3.js and ethers.js providers?

A ethers.js providers return BigNumber objects for numeric values, Web3.js returns strings or numbers.

B Web3.js providers automatically parse all contract events, ethers.js providers do not support events.

C Web3.js only supports HTTP providers, ethers.js only supports WebSocket providers.

D ethers.js requires manual ABI encoding for all contract calls, Web3.js does this automatically.

💡 Hint

Think about how each library handles large numbers from the blockchain.

ethers.js uses BigNumber objects to safely handle large integers, while Web3.js often returns strings or numbers which can lose precision.