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I have an ERC721 contract that mints an NFT. I added a function that mints multiple NFTs in one call. I tried returning the token ids from that function, but I cannot write the test code to get them.

Code:

//@dev mint token for user
  function mintToken() public payable returns(uint256) {
    require(_deactivated == false, "Token sale is deactivated");
    uint256 price = calculatePrice(msg.sender);
    require(msg.value >= price, "Insufficient ETH sent");

    //get token id
    _tokenIds.increment();
    uint256 tokenId = _tokenIds.current();

    //get uri
    string memory uri = randomUri();

    //mint and set uri
    _safeMint(msg.sender, tokenId);
    _setTokenURI(tokenId, uri);

    return tokenId;
  }

  //@dev mint multiple tokens for user
  function mintMultipleTokens(uint numberOfTokens) public payable returns(uint256[] memory) {
    require(_deactivated == false, "Token sale is deactivated");
    uint256 price = calculatePrice(msg.sender).mul(numberOfTokens);
    require(msg.value >= price, "Insufficient ETH sent");

    uint256[] memory tokenIds = new uint256[](numberOfTokens);
    for(uint index = 0; index < numberOfTokens; index++) {
      tokenIds[index] = mintToken();
    }
    return tokenIds;
  }

In my test, when I execute: const tokenIds = await token.connect(user3).mintMultipleTokens(numberOfTokens, {value: price}); I receive the following in tokenIds:

{
  hash: '0x8eab2571a37a17c481e9ede9c4d5eb99b410a743d9459dc1017e393500e67d72',
  type: 2,
  accessList: [],
  blockHash: '0xbbbda0142bbea68b403dc8bfc910cc037260dce00725aa1b8f6cd53c52f1c8b7',
  blockNumber: 42,
  transactionIndex: 0,
  confirmations: 1,
  from: '0x90F79bf6EB2c4f870365E785982E1f101E93b906',
  gasPrice: BigNumber { _hex: '0x3c1214ea', _isBigNumber: true },
  maxPriorityFeePerGas: BigNumber { _hex: '0x3b9aca00', _isBigNumber: true },
  maxFeePerGas: BigNumber { _hex: '0x3c895fd4', _isBigNumber: true },
  gasLimit: BigNumber { _hex: '0x01bad518', _isBigNumber: true },
  to: '0x09635F643e140090A9A8Dcd712eD6285858ceBef',
  value: BigNumber { _hex: '0x06f05b59d3b20000', _isBigNumber: true },
  nonce: 1,
  data: '0xe1dffcb90000000000000000000000000000000000000000000000000000000000000005',
  r: '0x6f8717d164eebdd6578218c4d8c5cce95e2244f7b6f218e15e846d32688bddb7',
  s: '0x616dc1810a7dcf4574e5254df4f144c0a6d1f30b3f9398637e28e0c899cfc768',
  v: 1,
  creates: null,
  chainId: 31337,
  wait: [Function (anonymous)]
}

How can I receive the ids array properly, so I can conclude my test?

I'm using Hardhat 2.6.8, Solidity 0.8.4, Node 17.0.1.

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1 Answer 1

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There is no way to get the return value of a transaction, directly. How this is usually done, is with one of two ways (or both):

  1. Emit events with the required data, and catch those events

  2. Get the required data through a view function. So the main function stores the result in a state variable(s) and that data is read in a separate view function. view functions can return data directly, since those are called with a local call, instead of a real transaction.

You can read more about this for example here: return function value instead of transaction receipt with web3

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  • I suspected it has something to do with view. But then, why can I get the value of mintToken (which is uint256) properly? it's not a view either? Commented Nov 15, 2021 at 21:51
  • My guess is that you're accidentally calling it as a local call, instead of a real transaction Commented Nov 15, 2021 at 21:52
  • So far, I've only been calling it in a Hardhat test. But it is a standard NFT mint function, and those always return the id? Commented Nov 15, 2021 at 21:53
  • Something else came to mind: each time an NFT gets minted, a Transfer event is fired. Is there a way to capture multiple Transfer events? That way, I won't need to return an array, as each even would include an id. Commented Nov 15, 2021 at 21:55
  • The ERC721 standard doesn't specify anything about minting, so its signature can be anything. If you have further questions, please post a new question or search for existing answers Commented Nov 15, 2021 at 21:56

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