Blockchain Developer Token Standards and Creation

Question 1

Which ERC-20 function returns the number of tokens a spender is still allowed to withdraw from an owner's account?

Accepted Answer

allowance(owner, spender)

Answer

allowance(owner, spender) returns the remaining amount the spender is permitted to withdraw.

Question 2

In ERC-20, what is the well-known race-condition risk associated with the approve() function?

Accepted Answer

A spender can front-run a changed approval and spend both the old and new amounts

Answer

Changing an existing non-zero allowance lets a spender front-run and use both old and new amounts.

Question 3

What does the ERC-20 decimals() function actually control?

Accepted Answer

Only the display formatting of token amounts off-chain

Answer

decimals() is purely informational for UI display; the contract stores integer values regardless.

Question 4

Which event must an ERC-20 token emit when tokens move between addresses?

Accepted Answer

Transfer(from, to, value)

Answer

The Transfer event is required on every token movement, including minting (from the zero address).

Question 5

Why is it best practice to emit a Transfer event from the zero address when minting ERC-20 tokens?

Accepted Answer

So off-chain indexers can track supply changes consistently

Answer

Emitting Transfer from address(0) lets explorers and indexers treat minting as a standard balance change.

Blockchain Practice Test