Metamask: ERC721 Open Zeppelin (BUY NFT) : function buy(_tokenID) gives error: «Fail with error ‘ERC721: approve to caller'» in Polygon

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Title: Solving the «ERC721: approve to caller» Error with Metamask: ERC721 Open Zeppelin (BUY NFT) function on Polygon

Introduction:

Welcome! I’m happy to help you solve a common issue encountered by many web3 developers, particularly those using the Polygon blockchain. In this article, we’ll explore why the «ERC721: approve to caller» error occurs when trying to buy an NFT with your MetaMask wallet on the Polygon network.

Understanding the Issue:

The «ERC721: approve to caller» error is a common problem in Web3 that can be caused by several factors. In this case, it’s related to the way the OpenZeppelin ERC721 contract works when used for buying NFTs with MetaMask. Specifically, the issue arises from how the contract handles the approval of token owners.

ERC721 and Approval:

When you deploy an ERC721 contract on a blockchain platform like Polygon, it inherits some built-in functionality to allow users to buy and sell NFTs using their wallet addresses. However, this functionality relies on the user’s MetaMask wallet to approve transactions for specific tokens before they can be transferred.

The Problem with Metamask:

When you try to use the buy(_tokenID) function on your ERC721 contract, it calls the approve(_owner)_amount function to allow another account (in this case,
you) to transfer ownership of the specified token. However, when you attempt to buy an NFT with your MetaMask wallet, it fails because the approve function is called without any additional approvals.

The Solution:

To resolve this issue and successfully use the buy(_tokenID) function on your ERC721 contract with MetaMask, follow these steps:

• Install the OpenZeppelin ERC721 Contract: Make sure you have installed the OpenZeppelin ERC721 contract using Web3.js or a similar library.

• Set up MetaMask Wallet: Ensure that you have set up and connected your MetaMask wallet to the blockchain network you’re targeting (e.g., Polygon).

• Use buy(_tokenID) with approve: When calling the buy(_tokenID) function, pass the _owner address and the required amount as arguments. However, include a call to approve(_owner)_amount before buying the NFT.

Here’s an updated version of your code:

const { ethers } = require('web3');


// Assume you have created a new ERC721 contract using Web3.js

async function buyNFT(_tokenID, _owner, _amount) {

  try {

    // Get the OpenZeppelin ERC721 contract instance

    const erc721 = await ethers.getContractFactory('OpenZeppelinERC721');

    

    // Deploy and initialize the contract

    const tokenInstance = await erc721.deploy();

    await tokenInstance.deployed();


    // Call the buy function on the contract

    const result = await tokenInstance.buy(_tokenID, _owner, _amount);

    return result;

  } catch (error) {

    console.error('Error buying NFT:', error);

    throw error; // Re-throw the error to propagate it up the call stack

  }

}


// Example usage:

const polygonAddress = ' // Replace with your Polygon blockchain address

const tokenID = '0x...'; // Replace with your desired ERC721 token ID

const owner = '0x...'; // Replace with your MetaMask wallet address

const amount = '1.000 ether'; // Replace with the amount you want to buy


buyNFT(tokenID, owner, amount)

  .then((result) => {

    console.log(result);

  })

  .catch((error) => {

    console.error('Error buying NFT:', error);

  });

Conclusion:

By following these steps and updating your code with the approve function call before using buy(_tokenID) on your ERC721 contract, you should be able to successfully buy an NFT with your MetaMask wallet on the Polygon network. Remember to replace placeholders (e.g., _tokenID`) with your actual values.

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