Easy Solidity-interview-questions (and answers).

Questions taken from - https://www.rareskills.io/post/solidity-interview-questions

Repository - https://github.com/typicalHuman/solidity-interview-questions

1. What is the difference between private, internal, public, and external functions?

private can only be used within the current contract, internal can also be used within derived contracts, public functions can be used inside and outside the current contract (by users and the contract itself), external functions can only be used by users.

Reference: https://docs.soliditylang.org/en/latest/contracts.html#state-variable-visibility

2. Approximately, how large can a smart contract be?

About 24,000 bytes and (4.7 million gas). According to EIP-170: In short, the point of this EIP is to minimize the overloading of nodes (especially light nodes) and to prevent possible DOS attacks.

Reference: https://docs.soliditylang.org/en/latest/cheatsheet.html#function-visibility-specifiers

3. What is the difference between create and create2?

CREATE is used in the default scenario when you deploy a smart contract via new for example. CREATE2 is also used to deploy smart contracts, but it has a special parameter - salt, which allows you to predetermine the future address of the contract.

Reference: https://vinta.ws/code/solidity-create-vs-create2.html

4. What major change with arithmetic happened with Solidity 0.8.0?

More secure arithmetic operations without explicitly using the SafeMath library (preventing underflows and overflows).

Reference: https://docs.soliditylang.org/en/latest/080-breaking-changes.html

5. What special CALL is required for proxies to work?

DELEGATECALL because it allows you to pass all the call context to the proxy contract and also to keep the storage states.

Reference: https://eips.ethereum.org/EIPS/eip-1967

6. Prior to EIP-1559, how do you calculate the dollar cost of an Ethereum transaction?

It used a simple gas price auction system and this system had some downsides such as: highly volatile gas price and delays for users.

Reference: https://eips.ethereum.org/EIPS/eip-1559

7. What are the challenges of creating a random number on the blockchain?

Because blockchain is a fully open system it means you can simulate and predict almost everything on it and all attempts to create true RNG were failed so for RNG on blockchain builders usually use some oracles like - https://docs.chain.link/vrf.

Reference: https://www.sitepoint.com/solidity-pitfalls-random-number-generation-for-ethereum/

8. What is the difference between a Dutch Auction and an English Auction?

In an English auction, the auctioneer sets a base price and bidders compete to see who can offer a higher price. In a Dutch auction, it's the other way round - the auctioneer sets a high price and goes lower and lower until someone agrees to accept the price.

Reference: https://saylordotorg.github.io/text_introduction-to-economic-analysis/s21-auctions.html

9. What is the difference between transfer and transferFrom in ERC20?

transfer only transfers the amount of tokens from msg.sender. transferFrom uses an allowance system, so for example if you want the contract to use your ERC20 token you can set an allowance for it to use your tokens so it can send the tokens on your behalf: transferFrom(you, someone, amount).

Reference: https://docs.openzeppelin.com/contracts/4.x/api/token/erc20#IERC20-transferFrom-address-address-uint256-

10. Which is better to use for an address allowlist: a mapping or an array? Why?

It's better to use a mapping, because it's cheaper to check an allowance, and the array can lead to a DOS attack if the number of users is too large.

Reference: https://ethereum.stackexchange.com/a/2597/99105

11. Why shouldn’t tx.origin be used for authentication?

This is because if the contract the user is calling is malicious - the malicious contract can act on behalf of the initial caller, but this will not happen if you use msg.sender instead, because in this situation the caller would be the malicious contract itself, not the user.

Reference: https://solidity-by-example.org/hacks/phishing-with-tx-origin/

12. What hash function does Ethereum primarily use?

Keccak-256

Reference: https://www.oreilly.com/library/view/mastering-ethereum/9781491971932/ch04.html#:~:text=Ethereum uses the Keccak-256,Institute of Science and Technology.

13. How much is 1 gwei of Ether?

1 * 10^-9 | 0.000000001 | 1 / 1,000,000,000

Reference: https://docs.soliditylang.org/en/v0.8.24/units-and-global-variables.html#ether-units

14. How much is 1 wei of Ether?

1 * 10^-18 | 0.000000000000000001 | 1 / 1,000,000,000,000,000,000

Reference: https://docs.soliditylang.org/en/v0.8.24/units-and-global-variables.html#ether-units

14. How much is 1 wei of Ether?

1 * 10^-18 | 0.000000000000000001 | 1 / 1,000,000,000,000,000,000

Reference: https://docs.soliditylang.org/en/v0.8.24/units-and-global-variables.html#ether-units

15. What is the difference between assert and require?

assert is usually used to check for internal errors (e.g. invariants), since this call is also used natively in the Solidity compiler itself, and also asserting panic errors should not happen in a well-tested contract, otherwise you have a bug in it. The require keyword is usually used to check for errors in user input/actions.

Reference: https://docs.soliditylang.org/en/v0.8.24/control-structures.html#panic-via-assert-and-error-via-require

16. What is a flash loan?

Flashloan is a DeFi operation where you can borrow some tokens for the time of executing a single transaction and return those tokens in the same transaction (most of the time with some fees on top).

Reference: https://docs.aave.com/faq/flash-loans

17. What is the check-effects pattern?

Check-Effects (and Interactions) is a structural pattern of code that allows you to avoid some cases of reentrancy, because first you do all the necessary checks, then you change a local state, and only then you interact with other contracts, otherwise you could be open to reentrancy attacks.

Reference: https://docs.soliditylang.org/en/latest/security-considerations.html#reentrancy

18. What is the minimum amount of Ether required to run a solo staking node?

32 ETH.

Reference: https://ethereum.org/en/staking/

19. What is the difference between fallback and receive?

The receive function is only triggered when an ETH event is received (with empty calldata), but the fallback function is triggered at any time if you call some data that didn't exist in the contract's ABI. And to make these functions work, you need to specify them explicitly.

Reference: https://docs.soliditylang.org/en/v0.8.24/contracts.html#receive-ether-function

20. What is reentrancy?

Reentrancy is a type of attack where the attacker re-enters your function and performs repetitive actions without any constraints (this action could be updating the state of the contract/draining tokens/creating votes and so on). Reentrancy is usually performed in functions where external interactions occur before the local state changes.

Reference: https://docs.soliditylang.org/en/latest/security-considerations.html#reentrancy

21. As of the Shanghai upgrade, what is the gas limit per block?

30 million gas.

Reference: https://ethereum.org/en/developers/docs/gas/#block-size

22. What prevents infinite loops from running forever?

There's a maximum cap for executing a transaction, and because the transaction requires an infinite amount of gas to execute - it can't be executed in the Ethereum ecosystem because the block has a cap of 30M gas per block.

Reference: https://ethereum.org/en/developers/docs/gas/#block-size

23. What is the difference between tx.origin and msg.sender?

tx.origin - initial user address that started execution of the transaction, msg.sender address that called current function, it can be another contract for example.

Reference: https://docs.soliditylang.org/en/v0.8.24/cheatsheet.html#block-and-transaction-properties

24. How do you send Ether to a contract that does not have payable functions, or a receive or fallback?

You can do this using the selfdestruct operation, because when the contract is destroyed all its value would be sent to the specified address.

Reference: https://docs.soliditylang.org/en/v0.8.24/introduction-to-smart-contracts.html#deactivate-and-self-destruct

25. What is the difference between view and pure?

view functions can read contract states, pure functions cannot, they can only use the input data provided.

Reference: https://docs.soliditylang.org/en/v0.8.24/contracts.html#state-mutability

26. What is the difference between transferFrom and safeTransferFrom in ERC721?

safeTransferFrom checks if the receiver is a contract and if it is a contract - it requires it to implement the IERC721Receiver interface, so you won't send your NFT to the contract without any way of getting it back, just transferFrom doesn't do this check.

Reference: https://docs.openzeppelin.com/contracts/4.x/api/token/erc721#IERC721-safeTransferFrom-address-address-uint256-

27. How can an ERC1155 token be made into a non-fungible token?

It's possible to set the maximum amount of each token to 1, so that each token has only 1 copy of itself. This can be done by setting the number of token copies when minting the token.

Reference: https://docs.openzeppelin.com/contracts/4.x/api/token/erc1155#ERC1155-_mint-address-uint256-uint256-bytes-

28. What is access control and why is it important?

Access control is a restriction mechanism that ensures that only users with certain roles/responsibilities can call some of the functions. For example, you don't want your users to have access to the `mint' function, because they can just mint a lot of tokens for themselves, defeating the purpose of NFT collection.

Reference: https://docs.openzeppelin.com/contracts/2.x/access-control

29. What does a modifier do?

Modifiers allow you to check some function call conditions before a function is actually executed.

Reference: https://docs.soliditylang.org/en/v0.8.24/contracts.html#function-modifiers

29. What does a modifier do?

Modifiers allow you to check some function call conditions before a function is actually executed.

Reference: https://docs.soliditylang.org/en/v0.8.24/contracts.html#function-modifiers

30. What is the largest value a uint256 can store?

2^256 - 1 (-1 because 1 space for the symbol)

Reference: https://ethereum.stackexchange.com/questions/58981/what-is-the-maximum-value-an-int-and-uint-can-store

31. What is variable and fixed interest rate?

Fixed interest rate - remains the same for the whole period (of loan for example), Variable interest fixed rate - can change during the period. Variable rates involve more risks, but they can save you more money or you will lose more money if some of the conditions would change (like economic ones or some internal policies).

Reference: https://www.investopedia.com/ask/answers/07/fixed-variable.asp