How to test a contract's ability to receive an ERC721 token?

Question

I'm testing my contract in Truffle. I enabled the contract to receive ERC721 tokens:

function onERC721Received(address, address _from, uint256 _tokenId, bytes calldata) external override returns(bytes4) {
    nftContract = ERC721(msg.sender);
    tokenId = _tokenId;
    tokenAdded = true;

    return 0x150b7a02;
}

Is there a way to simulate a token being sent to this contract using Mocha and Chai?

Answer 1

Outside of EVM (for example in a JS test), there's no way to check a return value of a transaction. Only its status (succeeded/reverted), emitted events (in your case non) and few other metadata. And you can also check return value of a call, as in the assert.equal statements.

contract('MyContract', () => {
    it('receives a token', async () => {
        const tx = await myContract.onERC721Received(
            '0x123',      // address
            '0x456',      // address _from
            1,            // uint256 _tokenId
            [0x01, 0x02]  // bytes calldata
        );

        assert.equal(tx.receipt.status, true); // tx succeeded

        assert.equal(await contract.nftContract, '0x123');
        assert.equal((await contract.tokenId).toNumber(), 1);
        assert.equal(await contract.tokenAdded, true);
    });
});

Docs:

• Truffle contract instead of Mocha describe - docs
• The receipt status - Truffle docs , Web3 docs (the link to Web3 in the Truffle docs is outdated)
• Not having to use .send() or .call() in Truffle, because it choses the tx or call automatically from the contract ABI - docs

Answer 2

I do test this using an ERC721 mock contract in my tests too. So I deploy both contracts and create new instances for them, then from the ERC721 call the mint function to the contract under test address. Then check the balance of ERC721 with:

ERC721.balanceOf(underTest.address, 1)