How to check the balance of a wallet within a contract and assign balance to a variable?

Question

I'm trying to require the number of contract NFTs in a wallet < declared uint256 variable in order to proceed with safemint function. I have the following code but I can't work out what is happening, or how to pass the result of balanceOf to a variable which I can later check against MAX_InWallet. Can anyone help please. Thanks

    uint256 MAX_SUPPLY = 20;
    uint256 MAX_InWallet = 5;

    function safeMint(address to, string memory uri) public {
        uint256 tokenId = _tokenIdCounter.current();
        require(tokenId <= MAX_SUPPLY, "Error, Minted Out");

        mapping (address => uint256) public view balanceOf; // week1 challenge
        require(balanceOf(msg.sender) < MAX_InWallet, "error"); // week1 challenge

        _tokenIdCounter.increment();
        _safeMint(to, tokenId);
        _setTokenURI(tokenId, uri);
    }

Answer 1

My two cents:

    import "@path/to/openzeppelin/contracts/token/IERC721.sol"
   
    uint256 MAX_SUPPLY = 20;
    uint256 MAX_InWallet = 5;
    IERC721 ThisNFT = IERC721(address(this));

    function safeMint(address to, string memory uri) public {
        uint256 tokenId = _tokenIdCounter.current();
        require(tokenId <= MAX_SUPPLY, "Error, Minted Out");
        require(ThisNFT.balanceOf(msg.sender) < MAX_InWallet, "5 tokens per wallet");

        _tokenIdCounter.increment();
        _safeMint(to, tokenId);
        _setTokenURI(tokenId, uri);
    }

You should learn about ERC721 and how to create a instance for it inside the contract :)

Answer 2

This can be simplified a bit further since you're already inheriting from an ERC-721 to the following:

    uint256 MAX_SUPPLY = 20;
    uint256 MAX_InWallet = 5;

    function safeMint(address to, string memory uri) public {
        uint256 tokenId = _tokenIdCounter.current();
        require(tokenId <= MAX_SUPPLY, "Error, Minted Out");

        // you can change the require statement to this instead
        require(balanceOf(to) <= MAX_InWallet, "Exceeds max wallet count");

        _tokenIdCounter.increment();
        _safeMint(to, tokenId);
        _setTokenURI(tokenId, uri);
    }

recap:

• removed the import statement of IERC721
• removed setting up ThisNFT variable
• simplified the require check