Metamask: Checking ERC721 token approvals in a for loop works in Hardhat Script but fails in React

Metamask Issue: Checking ERC721 Token Approvals in Hardhat Script vs React

When working with the MetaMask API and Solidity contracts, there are certain edge cases that can lead to unexpected behavior. One common issue is when checking token approvals on ERC721 tokens using a for loop in a Hardhat script versus a React application.

In this article, we’ll explore the issue and provide solutions to resolve it.

Hardhat Script Issue

In a Hardhat script, you can use the transfer function to transfer ownership of an ERC721 token. When checking token approvals on these contracts using a for loop, you may encounter issues with the approval status being updated correctly.

const contractArr = ['0x1234567890abcdef'];


modifier isApproved(address[] memory contractArr) {

    require(contractArr.length > 0, "Length Zero");

    for (let i = 0; i < contractArr.length; i++) {

        const tokenAddress = contractArr[i];

        contract[tokenAddress].approve(address.from, address.value);

    }

}

The issue lies in the fact that when a transfer is made using transfer function, it returns true immediately after the transaction is processed. However, in a for loop, you need to check if the approval status has been updated correctly.

React Issue

In a React application, checking token approvals on ERC721 tokens can be more complex due to the asynchronous nature of the API calls.

import { contract } from './ contracts/ERC721.sol';

import Web3 from 'web3';


const contractArr = ['0x1234567890abcdef'];


function isApproved(address) {

    const web3 = new Web3(window.ethereum);

    return contract.ERC721.isApproved(contract.address, address.value);

}

In this React example, the isApproved function uses the contract object from the Solidity contracts to check if the token has been approved.

Solution

To resolve these issues in both Hardhat scripts and React applications, you can use a different approach:

• Use the transfer function with an event emitter: Instead of checking the approval status directly, use the eventTransferComplete event emitted by the MetaMask API to update your contract’s state.

const contractArr = ['0x1234567890abcdef'];


modifier isApproved(address[] memory contractArr) {

    require(contractArr.length > 0, "Length Zero");


    for (let i = 0; i < contractArr.length; i++) {

        const tokenAddress = contractArr[i];

        eventTransferComplete(tokenAddress).emit('approval', { status: true });

        // Update the contract state

        contract[tokenAddress].approve(address.from, address.value);

    }

}

• Use a callback function in the transfer event: You can also pass a callback function to the transfer event emitted by MetaMask that will be called when the approval status is updated.

const contractArr = ['0x1234567890abcdef'];


eventTransferComplete(tokenAddress).on('approval', (status) => {

    if (status.status === true) {

        // Update the contract state

        contract[tokenAddress].approve(address.from, address.value);

    }

});

By using these approaches, you can ensure that your contracts are checked for token approvals correctly in both Hardhat scripts and React applications.

Conclusion

When working with the MetaMask API and Solidity contracts, it’s essential to be aware of potential issues like checking token approvals on ERC721 tokens. By understanding the underlying mechanics of the transfer function and using event emitters or callback functions, you can resolve these issues in both Hardhat scripts and React applications.

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