IERC721Enumerable

IERC721Enumerable is an optional extension of the ERC-721 standard that provides a standardized way to enumerate NFTs managed by a contract. It defines a separate interface, IERC721Enumerable, that extends the core IERC721 interface. Implementing this extension enables external accounts and applications to list all tokens and to list tokens owned by a specific address. Because enumerability requires maintaining additional on-chain data, not all ERC-721 tokens implement this extension; it increases minting, transfer, and storage costs.

The interface declares three view-only functions. totalSupply() returns the total number of tokens tracked by the contract. tokenOfOwnerByIndex(address owner, uint256 index) returns the tokenId owned by the given address at a particular index within that owner's collection. tokenByIndex(uint256 index) returns the tokenId at a given index in the global sequence of all tokens. These functions enable predictable iteration over tokens, both globally and per owner.

Usage in applications is straightforward: to enumerate all tokens, a caller can iterate from 0 to totalSupply() - 1 and call tokenByIndex(i) for each index. To display the tokens owned by a particular address, iterate from 0 to balanceOf(owner) - 1 and call tokenOfOwnerByIndex(owner, i). This capability is commonly used by wallets, marketplaces, and NFT explorers to present holdings and listings.

Implementation notes: when a contract supports IERC721Enumerable, it must maintain internal structures that reflect current ownership and the global token set, updating them on mints, burns, and transfers. Because enumeration imposes additional data and gas costs, developers choose whether to implement the extension based on the needs of their ecosystem.