Blockchain Developer Token Standards and NFTs 3

Question 1

Which ERC-20 function approves a spender to transfer tokens on the owner's behalf?

Accepted Answer

approve

Answer

approve(spender, amount) authorizes a spender to move up to amount tokens via transferFrom.

Question 2

What known issue affects the classic ERC-20 approve function when changing a non-zero allowance?

Accepted Answer

The approve race condition (front-running) double-spend risk

Answer

Changing an existing allowance can be front-run, letting a spender use both old and new allowances.

Question 3

Which standard adds gasless approvals to ERC-20 tokens via off-chain signatures (permit)?

Accepted Answer

ERC-2612

Answer

ERC-2612 introduces permit, allowing approvals through signed messages without a separate on-chain transaction.

Question 4

What does the ERC-20 decimals function represent?

Accepted Answer

The number of decimal places used to display token amounts

Answer

decimals indicates how to scale the smallest unit; e.g., 18 decimals means 1 token = 10^18 base units.

Question 5

Why can sending ERC-20 tokens directly to a contract not expecting them cause loss?

Accepted Answer

Plain transfer has no callback, so the contract may not register the deposit

Answer

ERC-20 transfer does not notify the recipient contract, so tokens can be stranded if the contract isn't designed to handle them.

Blockchain Practice Test