Check TokenStandard of specify contract address

[defi0x1]

Describe your Issue

Hi author @r and everyone here.
Currently, i am researching about ERC20, ERC721 token and using this library. Ethers.js is great library.
But i am finding some way to check token standard (like ERC20, ERC721 or ERC-1155). Could anyone help me how to check it.

Thanks !

This is an exception issue

• I understand this form is for exceptional issues and believe my issue qualifies.

[zemse]

Hey @docongminh, there isn't a straight forward way to verify if a contract adheres to a standard without manually looking at the source code.

However, there are some attempts to help this programmatically e.g. the ERC165 standard. OpenZeppelin has some tools for the same, ERC165Checker oz docs (though it can't be trusted as a malicious contract can say that it adheres to a standard say ERC721, but in reality it doesn't).

[defi0x1]

@zemse Hi. But why can NFT marketplaces (like opensea,..) detect standard of each nft listing on its ?

[zemse]

If the contract is verified on etherscan, you can have the best bet by looking at ABI. However if a contract is not verified then it's more challenging to tell.

One can add multiple checks, e.g. in case of etherscan, they just detect a contract is ERC20 or ERC721 by events. You can notice that when you deploy an ERC20 contract, Etherscan does not immediately recognise it as ERC20, but when the first transfer or approve takes place, it acknowledges it as ERC20.

It depends on what you need, if the contract seems to do that then you can just assume it adheres to the standard (knowing that if it doesn't it should not cause loss). For e.g. if your project involves transferring tokens, a contract address had successful transfer calls in history. But AFAIK there isn't a 100% sure way to determine if an address exactly adheres to the ERC20 spec.

[defi0x1]

@zemse Thank you. I am thinking that. We can use decimal as a way to check a token is ERC20 or NFT. You know, with NFT or semi-NFT decimals alway to be zero. Do you think about this ?

[zemse]

decimals() can help with ERC20 but for NFT I don't think it's in the standard. You can try calling the method supportsInterface("0x80ac58cd") (note that in the EIP in the EIP the interface id is mentioned), if it returns true it should very likely be an NFT, if it's false then it's likely not an NFT, if it throws then not sure.