Listening to events on frontend in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to listen to the 'Transfer' event from a smart contract.

Blockchain / Solidity

contract.on('Transfer', (from, to, amount) => {
  console.log('Transfer from', from, 'to', to, 'amount', [1]);
});

Drag options to blanks, or click blank then click option'

Aamount

Bto

Cfrom

Devent

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'to' or 'from' instead of 'amount' to log the transfer value.

The 'amount' parameter holds the value transferred in the event. We log it to see the transfer amount.

2fill in blank

medium

Complete the code to stop listening to the 'Approval' event.

Blockchain / Solidity

contract.[1]('Approval');

Drag options to blanks, or click blank then click option'

AremoveAllListeners

Bon

Clisten

Demit

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'on' instead of 'removeAllListeners' to stop listening.

To stop listening to an event, we use 'removeAllListeners' with the event name.

3fill in blank

hard

Fix the error in the code to correctly listen to the 'Deposit' event and log the amount.

Blockchain / Solidity

contract.on('Deposit', (user, amount) => {
  console.log('User deposited:', [1]);
});

Drag options to blanks, or click blank then click option'

Adeposit

Buser

Cevent

Damount

Attempts:

3 left

💡 Hint

Common Mistakes

Logging 'user' instead of 'amount' for the deposit value.

The 'amount' parameter holds the deposit value, so we log it to show how much was deposited.

4fill in blank

hard

Fill both blanks to create a filter for the 'Transfer' event where the sender is a specific address.

Blockchain / Solidity

const filter = contract.filters.Transfer([1], [2]);

Drag options to blanks, or click blank then click option'

A'0x1234567890abcdef1234567890abcdef12345678'

Bnull

C'0xabcdefabcdefabcdefabcdefabcdefabcdefabcd'

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'undefined' or a wrong address in the blanks.

The first blank is the sender address to filter by. The second blank is 'null' to ignore the receiver address.

5fill in blank

hard

Fill all three blanks to listen to the 'Approval' event, filter by owner address, and log the spender and value.

Blockchain / Solidity

const filter = contract.filters.Approval([1], null);

contract.on(filter, (owner, spender, value) => {
  console.log('Spender:', [2], 'Value:', [3]);
});

Drag options to blanks, or click blank then click option'

A'0xabcdefabcdefabcdefabcdefabcdefabcdefabcd'

Bspender

Cvalue

D'0x1234567890abcdef1234567890abcdef12345678'

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up owner and spender addresses or logging wrong variables.

The first blank is the owner address to filter by. Then we log the 'spender' and 'value' parameters from the event.

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