Token Standards and NFTs Flashcards
7 cards from real Blockchain Technology practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Token Standards and NFTs flashcards as text
Which ERC-20 function approves a spender to transfer tokens on the owner's behalf?
Answer: approve
approve(spender, amount) authorizes a spender to move up to amount tokens via transferFrom.
What known issue affects the classic ERC-20 approve function when changing a non-zero allowance?
Answer: The approve race condition (front-running) double-spend risk
Changing an existing allowance can be front-run, letting a spender use both old and new allowances.
Which standard adds gasless approvals to ERC-20 tokens via off-chain signatures (permit)?
Answer: ERC-2612
ERC-2612 introduces permit, allowing approvals through signed messages without a separate on-chain transaction.
What does the ERC-20 decimals function represent?
Answer: The number of decimal places used to display token amounts
decimals indicates how to scale the smallest unit; e.g., 18 decimals means 1 token = 10^18 base units.
Why can sending ERC-20 tokens directly to a contract not expecting them cause loss?
Answer: Plain transfer has no callback, so the contract may not register the deposit
ERC-20 transfer does not notify the recipient contract, so tokens can be stranded if the contract isn't designed to handle them.
Which standard, ERC-777, adds hooks that notify contracts on token sends and receives?
Answer: ERC-777
ERC-777 introduces tokensReceived/tokensToSend hooks via ERC-1820 to react to transfers.
What security concern do ERC-777 receive hooks introduce that plain ERC-20 does not?
Answer: Reentrancy via the callback during transfer
Because hooks hand control to the recipient mid-transfer, they can enable reentrancy attacks if unguarded.