Mastering Blockchain Programming with Solidity

The _exists internal function

_exists() is an internal function that is used to check that the given tokenId exists in the contract.

Let's look at the _exists() function code:

function _exists(uint256 tokenId) internal view returns (bool) { address owner = _tokenOwner[tokenId];    return owner != address(0);}

As you can see in the preceding code, the provided tokenId is checked in the _tokenOwner mapping. If the owner is address(0), the function returns false; otherwise, it returns true.