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.