Listening to events on frontend in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when listening to a simple event?

Consider the following code snippet that listens to a blockchain event using ethers.js. What will be logged to the console when the event is emitted with value 42?

Blockchain / Solidity

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

const provider = new ethers.providers.JsonRpcProvider();
const contractAddress = '0x1234567890abcdef1234567890abcdef12345678';
const abi = [
  'event ValueChanged(uint256 newValue)'
];

const contract = new ethers.Contract(contractAddress, abi, provider);

contract.on('ValueChanged', (newValue) => {
  console.log(`New value is: ${newValue.toString()}`);
});

// Assume event ValueChanged(42) is emitted by the contract

A New value is: 42

B TypeError: newValue.toString is not a function

C New value is: undefined

D New value is: 0x2a

💡 Hint

Remember that ethers.js BigNumber objects have a toString() method to convert to readable strings.

The event listener receives a BigNumber object representing the value 42. Calling toString() converts it to the string '42'.

❓ Predict Output

intermediate

What happens if you forget to remove an event listener?

Given the following code, what will be the output after emitting the event twice?

Blockchain / Solidity

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

const provider = new ethers.providers.JsonRpcProvider();
const contractAddress = '0xabcdefabcdefabcdefabcdefabcdefabcdefabcd';
const abi = [
  'event DataUpdated(string data)'
];

const contract = new ethers.Contract(contractAddress, abi, provider);

function onDataUpdated(data) {
  console.log(`Data updated: ${data}`);
}

contract.on('DataUpdated', onDataUpdated);

// Event DataUpdated('First') emitted
// Event DataUpdated('Second') emitted

A

Data updated: First
Data updated: First

B

Data updated: First
Data updated: Second
Data updated: Second

C

Data updated: First
Data updated: Second

D No output because listener is not removed

💡 Hint

Think about how many times the listener is attached and if it is removed.

The listener is attached once and listens to both events, so it logs both updates once each.

🔧 Debug

advanced

Why does this event listener not log anything?

Look at the code below. The event Transfer is emitted on the contract, but the console never logs anything. What is the reason?

Blockchain / Solidity

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

const provider = new ethers.providers.JsonRpcProvider();
const contractAddress = '0xfeedfeedfeedfeedfeedfeedfeedfeedfeedfeed';
const abi = [
  'event Transfer(address indexed from, address indexed to, uint256 value)'
];

const contract = new ethers.Contract(contractAddress, abi, provider);

contract.on('transfer', (from, to, value) => {
  console.log(`Transfer from ${from} to ${to} of ${value.toString()}`);
});

A No output because the event is not indexed

B No output because event name is case-sensitive and should be 'Transfer' not 'transfer'

C No output because the callback parameters are in wrong order

D No output because the provider is not connected to the right network

💡 Hint

Check the event name spelling and capitalization.

Event names in ethers.js are case-sensitive. Using 'transfer' instead of 'Transfer' means the listener never matches the event.

📝 Syntax

advanced

Which option correctly listens to multiple events with one callback?

You want to listen to both Deposit and Withdrawal events from a contract and log their details with one function. Which code snippet is correct?

A contract.on('Deposit', (amount) => { console.log(amount); }); else contract.on('Withdrawal', (amount) => { console.log(amount); });

B contract.on('Deposit' || 'Withdrawal', (amount) => { console.log(amount); });

C contract.on(['Deposit', 'Withdrawal'], (event) => { console.log(event); });

D

contract.on('Deposit', (amount) => { console.log(amount); });
contract.on('Withdrawal', (amount) => { console.log(amount); });

💡 Hint

Think about how to attach multiple listeners properly.

Option D attaches separate listeners for each event. Option D is invalid syntax. Option D uses logical OR incorrectly. Option D has invalid syntax.

🚀 Application

expert

How to update UI state on event emission in React frontend?

You have a React app that listens to a blockchain event BalanceChanged. You want to update the displayed balance whenever the event fires. Which approach correctly updates the React state?

Blockchain / Solidity

import React, { useEffect, useState } from 'react';
import { ethers } from 'ethers';

const contractAddress = '0xabcabcabcabcabcabcabcabcabcabcabcabcabca';
const abi = [
  'event BalanceChanged(address indexed user, uint256 newBalance)'
];

export default function Balance() {
  const [balance, setBalance] = useState('0');

  useEffect(() => {
    const provider = new ethers.providers.Web3Provider(window.ethereum);
    const contract = new ethers.Contract(contractAddress, abi, provider);

    // Fill in event listener here

    return () => {
      // Clean up listener
    };
  }, []);

  return <div>Balance: {balance}</div>;
}

A

contract.on('BalanceChanged', (user, newBalance) => {
  setBalance(newBalance.toString());
});

return () => {
  contract.off('BalanceChanged');
};

B

contract.on('BalanceChanged', (user, newBalance) => {
  setBalance(newBalance);
});

return () => {
  contract.off('BalanceChanged');
};

C

contract.on('BalanceChanged', (user, newBalance) => {
  balance = newBalance.toString();
});

return () => {
  contract.removeListener('BalanceChanged');
};

D

contract.on('BalanceChanged', (user, newBalance) => {
  setBalance(parseInt(newBalance));
});

return () => {
  contract.off('BalanceChanged');
};

💡 Hint

Remember to use the React state setter and convert BigNumber to string.

Option A correctly uses setBalance with newBalance.toString() and cleans up the listener with contract.off. Option A tries to assign state variable directly which won't update UI. Option A passes BigNumber directly without conversion. Option A uses parseInt which may lose precision.

Practice

1. What is the main purpose of listening to blockchain events on the frontend?

easy

A. To compile smart contracts

B. To send transactions to the blockchain

C. To react instantly to changes happening on the blockchain

D. To mine new blocks

Solution

1. Step 1: Understand what events represent

   Events are signals emitted by smart contracts when something important happens.

2. Step 2: Connect event listening to frontend behavior

   Listening to these events lets the frontend update immediately without waiting for manual refresh.

3. Final Answer:

   To react instantly to changes happening on the blockchain -> Option C

4. Quick Check:

   Listening to events = instant frontend updates [OK]

Hint: Events notify frontend about blockchain changes instantly [OK]

Common Mistakes:

• Confusing event listening with sending transactions
• Thinking events compile contracts
• Believing events mine blocks

2. Which of the following is the correct way to start listening to an event named Transfer using ethers.js?

easy

A. contract.subscribe('Transfer', (from, to, amount) => { console.log(from, to, amount); });

B. contract.listen('Transfer', (from, to, amount) => { console.log(from, to, amount); });

C. contract.addEventListener('Transfer', (from, to, amount) => { console.log(from, to, amount); });

D. contract.on('Transfer', (from, to, amount) => { console.log(from, to, amount); });

Solution

1. Step 1: Recall ethers.js event listening syntax

   In ethers.js, the method to listen to events is on.

2. Step 2: Match the correct method with parameters

   The correct syntax is contract.on(eventName, callback).

3. Final Answer:

   contract.on('Transfer', (from, to, amount) => { console.log(from, to, amount); }); -> Option D

4. Quick Check:

   ethers.js uses .on() for events [OK]

Hint: Use .on() method to listen to events in ethers.js [OK]

Common Mistakes:

• Using .listen() which does not exist in ethers.js
• Using .addEventListener() which is DOM method, not ethers.js
• Using .subscribe() which is not ethers.js syntax

3. Given the following code snippet, what will be logged when the Deposit event is emitted with arguments user='0x123' and amount=100?

contract.on('Deposit', (user, amount) => {
  console.log(`User ${user} deposited ${amount} tokens`);
});

medium

A. User undefined deposited undefined tokens

B. User 0x123 deposited 100 tokens

C. User [object Object] deposited 100 tokens

D. SyntaxError: Unexpected token

Solution

1. Step 1: Understand event callback parameters

   The callback receives event arguments in order: user and amount.

2. Step 2: Check the console.log output format

   The template string inserts user and amount correctly into the message.

3. Final Answer:

   User 0x123 deposited 100 tokens -> Option B

4. Quick Check:

   Event args used correctly = correct message [OK]

Hint: Event callback parameters match emitted event arguments [OK]

Common Mistakes:

• Assuming parameters are undefined if not destructured
• Confusing objects with strings in output
• Expecting syntax errors from correct code

4. Identify the error in this code snippet that listens to an event and suggest the fix:

contract.on('Approval', (owner, spender, value) => {
  console.log(owner, spender, value);
});

// Later in the code
contract.off('Approval');

medium

A. The off method requires the same callback function to remove the listener

B. The event name should be lowercase 'approval' instead of 'Approval'

C. The callback function must be async to listen to events

D. The off method does not exist in ethers.js

Solution

1. Step 1: Understand how to remove event listeners in ethers.js

   To remove a listener, you must pass the exact callback function used when adding it.

2. Step 2: Check the code's off usage

   The code calls contract.off('Approval') without the callback, so it won't remove the listener.

3. Final Answer:

   The off method requires the same callback function to remove the listener -> Option A

4. Quick Check:

   Remove listener = same callback needed [OK]

Hint: Pass same callback to .off() to remove listener [OK]

Common Mistakes:

• Calling .off() without callback removes nothing
• Changing event name case incorrectly
• Thinking callback must be async

5. You want to listen to a MessageSent event on your frontend and update the UI only for messages sent by the current user. Which approach correctly filters events before updating the UI?

hard

A. Listen to all MessageSent events and inside the callback check if sender === currentUser before updating UI

B. Use contract.on('MessageSent', callback, { filter: sender === currentUser }) to filter events automatically

C. Filter events by modifying the contract ABI to include only currentUser events

D. Listen to events only once using contract.once and update UI

Solution

1. Step 1: Understand event filtering on frontend

   ethers.js does not support filtering events by passing filter objects in on method directly.

2. Step 2: Use callback logic to filter events

   Listen to all events, then check inside the callback if the sender matches the current user before updating UI.

3. Final Answer:

   Listen to all MessageSent events and inside the callback check if sender === currentUser before updating UI -> Option A

4. Quick Check:

   Filter inside callback = correct approach [OK]

Hint: Filter events inside callback, not in .on() parameters [OK]

Common Mistakes:

• Trying to filter events via .on() parameters
• Modifying ABI to filter events (not possible)
• Using .once() which listens only once, not continuously