balanceOf

The balanceOf function is a standard method defined in token smart contracts, most notably within the ERC-20 and ERC-721 standards on Ethereum and other EVM-compatible chains. It is a read-only or view function that takes a single parameter—an address—and returns the token balance associated with that address as an unsigned integer. This function is fundamental for wallets, explorers, and decentralized applications (dApps) to display user holdings, verify ownership, and enforce access controls. Its standardized signature is function balanceOf(address _owner) public view returns (uint256 balance).

From a technical perspective, balanceOf queries a mapping data structure inside the smart contract, typically declared as mapping(address => uint256) private _balances;. When called, it does not modify the blockchain state or require gas from the caller, making it free to execute on most nodes. This function is the primary mechanism for off-chain systems and indexers to aggregate and report token distribution data. Its reliability and standardization are critical for the composability of the DeFi ecosystem, allowing protocols to trustlessly verify collateral or stake.

Common use cases include checking a wallet's token balance in a user interface, verifying if an address holds a specific Non-Fungible Token (NFT) for gated access, and enabling smart contracts to perform logic based on a user's holdings (e.g., requiring a minimum balance to vote in a DAO). For NFTs (ERC-721), the return value is either 0 or 1 for a given token ID, indicating ownership. It is often paired with the transfer function, which subsequently updates the internal balances mapping that balanceOf reads from.

While seemingly simple, the balanceOf function's implementation must be secure and accurate, as it is a trust anchor for financial logic. Malicious or incorrect implementations can lead to severe vulnerabilities, such as allowing unauthorized transfers or incorrect balance reporting. Developers must ensure it integrates correctly with other state-changing functions to maintain invariants, like the total supply always equaling the sum of all balances. Audits frequently focus on the consistency of the balances mapping manipulated by this function.

The balanceOf function is defined by the ERC-20 standard for fungible tokens and the ERC-721 standard for non-fungible tokens (NFTs). Its primary purpose is on-chain state querying, allowing any external caller—such as a wallet, decentralized application (dApp), or another smart contract—to programmatically check how many units of a token a specific Ethereum address holds. The function signature is typically balanceOf(address _owner) public view returns (uint256), where _owner is the address being queried and the return value is an integer representing the balance.

For fungible ERC-20 tokens like DAI or USDC, balanceOf returns a simple count of token units, where 1 unit often equals 1 * 10^18 of the token's smallest denomination (wei). For non-fungible ERC-721 tokens, the function's behavior is identical in signature but different in semantics: it returns the number of distinct NFT token IDs owned by the address, not the sum of their values. This function is a view function, meaning it does not modify the blockchain state, requires no gas fee to call, and is executed locally by an Ethereum node.

The balanceOf function is fundamental to blockchain interoperability and composability. Wallets use it to display user holdings, decentralized exchanges (DEXs) use it to check liquidity provider stakes, and lending protocols use it to determine a user's collateral balance. Its reliability depends on the correct implementation of the underlying token contract's internal balance tracking, usually a mapping such as mapping(address => uint256) private _balances;. Understanding balanceOf is essential for developers building any application that interacts with tokenized assets on Ethereum and compatible EVM chains like Polygon, Arbitrum, and BNB Smart Chain.