The tutorial for deploying NFTs has issues.

[creulean]

Today I deployed the NFT according to the tutorial in Soneium's documentation. I only changed the name of the NFT, and a problem occurred.

My code looks like this:

// SPDX-License-Identifier: MIT
// Compatible with OpenZeppelin Contracts ^5.0.0
pragma solidity ^0.8.22;

import {ERC721} from "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import {ERC721Enumerable} from "@openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol";
import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";

contract Snow is ERC721, ERC721Enumerable, Ownable {
    uint256 private _nextTokenId;

    constructor(address initialOwner)
        ERC721("Snow", "MTK")
        Ownable(initialOwner)
    {}

    function safeMint(address to) public {
        uint256 tokenId = _nextTokenId++;
        _safeMint(to, tokenId);
        return tokenId;    // There is a problem here.
    }

    // The following functions are overrides required by Solidity.

    function _update(address to, uint256 tokenId, address auth)
        internal
        override(ERC721, ERC721Enumerable)
        returns (address)
    {
        return super._update(to, tokenId, auth);
    }

    function _increaseBalance(address account, uint128 value)
        internal
        override(ERC721, ERC721Enumerable)
    {
        super._increaseBalance(account, value);
    }

    function supportsInterface(bytes4 interfaceId)
        public
        view
        override(ERC721, ERC721Enumerable)
        returns (bool)
    {
        return super.supportsInterface(interfaceId);
    }
}

After running the command npx hardhat test --network hardhat, the following issue occurred:

TypeError: Different number of arguments in return statement than in returns declaration.
  --> contracts/NFT.sol:20:9:
   |
20 |         return tokenId;
   |         ^^^^^^^^^^^^^^


Error HH600: Compilation failed

For more info go to https://hardhat.org/HH600 or run Hardhat with --show-stack-traces

The issue with the code is the missing return value declaration for the safeMint function. So I removed the return value and it ran successfully. The code is:

function safeMint(address to) public {
    uint256 tokenId = _nextTokenId++;
    _safeMint(to, tokenId); // ✅ Just cast, no need to return.
}

or

function safeMint(address to) public returns (uint256) { // ✅ Declare return uint256
uint256 tokenId = _nextTokenId++;
_safeMint(to, tokenId);
return tokenId; // ✅ return tokenId
}

After running npx hardhat test --network hardhat again, return:

Compiled 18 Solidity files successfully (evm target: paris).


  0 passing (0ms)

I don't know why I have this problem, but after using the two solutions above, the error disappeared.

I'm not sure if my modifications will affect the subsequent code. I hope you can give me some guidance on why this problem occurred and whether my issue can be resolved this way.

[soneium-official]

Dear @creulean,

Thank you for trying out our NFT tutorial and for reaching out. You are absolutely right—the safeMint method should not have a return value, and your solution is correct as well.

However, your code differs from what is written in the Soneium documentation. You can find the official code here. Specifically, the safeMint method is defined as follows:

    /// @dev the function can be called by anyone
    function safeMint(address to) public {
        uint256 tokenId = _nextTokenId++;
        _safeMint(to, tokenId);
    }

This means the erroneous line in your code is not present in our documentation. Could it be that you copied it from another source?
In any case, I’m glad to hear that the tutorial is working for you now! If you have any further questions, feel free to ask.