Listening to events on frontend in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to listen to events on the frontend in blockchain applications?

It means the frontend waits and reacts when the blockchain sends a message about something that happened, like a transaction or a change in data.

beginner

Which JavaScript object is commonly used to listen to blockchain events in a frontend app?

The Contract object from libraries like ethers.js or web3.js is used to listen to smart contract events.

intermediate

How do you start listening to an event named Transfer using ethers.js?

You use contract.on('Transfer', (from, to, amount) => { ... }) to run code whenever the Transfer event happens.

intermediate

Why is listening to events better than polling the blockchain repeatedly?

Listening to events is efficient because it only reacts when something happens, saving time and resources compared to checking the blockchain all the time.

intermediate

What should you do to stop listening to an event when it is no longer needed?

You call contract.off('EventName', callback) to remove the event listener and avoid memory leaks or unwanted updates.

Which library is commonly used to listen to blockchain events on the frontend?

A Express

B ethers.js

C Node.js

D React

ethers.js is a popular library to interact with Ethereum blockchain and listen to smart contract events.

What method do you use to listen to an event on a contract object?

A contract.watch()

B contract.listen()

C contract.on()

D contract.subscribe()

The on method is used to add an event listener on a contract.

What is the main advantage of listening to events instead of polling the blockchain?

A It reacts only when events happen

B It requires manual refresh

C It uses more CPU

D It slows down the app

Listening to events triggers code only when something happens, making it efficient.

How do you stop listening to an event in ethers.js?

A contract.off()

B contract.removeListener()

C contract.stop()

D contract.end()

The off method removes an event listener.

Which of these is NOT a reason to listen to blockchain events on the frontend?

A To update UI when data changes

B To react to smart contract actions

C To save resources by avoiding constant checks

D To manually refresh the page

Listening to events automates updates; manual refresh is not needed.

Explain how you would listen to a smart contract event on the frontend and why it is useful.

Think about how the frontend waits for messages from the blockchain.

Describe the steps to stop listening to an event and why it is important.

Consider what happens if you keep listening forever.

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