How to make sure that a token exist?

Question

I want to sell an NFT within my smart contract. But first, I want to check with a require statement if the token (NFT) exists or not before moving on to the next steps. I have tried require(ownerOf(tokenId), "Token doesn't exists."); but Remix says:

from solidity:
TypeError: No matching declaration found after argument-dependent lookup.
   --> contracts/charMinter.sol:108:9:
    |
108 |         require(ownerOf(tokenId), "Token doesn't exists.");
    |         ^^^^^^^
Note: Candidate: function require(bool)
Note: Candidate: function require(bool, string memory)

I also tried require(tokenURI(tokenId), "Token doesn't exists."); but I get the same error as above. Is there a way to check if a token exists or not?

Answer 1

You can use _exists(uint tokenId) function inheritanced from ERC721 smart contract.

You must to change your original condition like this:

require(_exists(tokenId), "Token doesn't exists.");

You can read more about this function here.

Example of smart contract that use this function:

// SDPX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";

contract Test is ERC721 {

    constructor() ERC721("TestToken", "TT") {
        _mint(msg.sender, 0);
    }

    function checkIfTokenExist(uint _tokenId) external view returns(address) {
        require(_exists(_tokenId), "Error!");
        return ownerOf(_tokenId);
    }
}

Answer 2

Here's an implementation example that is OpenZeppelin 5.x compliant:

function tokenURI(uint256 tokenId) public view override returns (string memory) {
    require(_ownerOf(tokenId) != address(0), "ERC721: URI query for nonexistent token");

    string memory _tokenURI = _tokenURIs[tokenId];
    if (bytes(_tokenURI).length > 0) {
        return _tokenURI;
    }

    return bytes(baseURI).length > 0
        ? string(abi.encodePacked(baseURI, tokenId.toString(), baseExtension))
        : "";
}