Can I get an ERC20 balance n blocks back in time?

Question

By calling a running Ethereum node through the getBalance API, you can get the Ethereum balance N blocks back as explained here. This is useful if you would only like to trust a balance which has been sufficiently confirmed by the network.

Is the same possible for ERC20 tokens? Does there exist a service that can show me what the token balance for a given address was 5 blocks ago?

Answer 1

Yes, you can do this. With web3.js 0.2x.x, note from https://github.com/ethereum/wiki/wiki/JavaScript-API#contract-methods that all calls to contracts take an optional block number.

So the following will get the balance of an account at a specific block number:

tokenContract.balanceOf(account, {}, blockNumber);

I believe the syntax for web3.js 1.0.0-beta is something like this, but double check the documentation:

tokenContract.methods.balanceOf(account).call({}, blockNumber);

If you're using raw JSON-RPC calls, eth_call takes a block number as a second parameter too.

Answer 2

As you mentioned, to get the ETH balance of an address you need to do this:

eth.getBalance("<ADDRESS_HERE>", <BLOCK_NUMBER>);

If you want to get the balance of an ERC20 token, you need to make a call to that token's contract. To do this you will need three things:

• The token contract address.
• Your account address.
• The token contract’s ABI.

So, you need instantiate the contract ABI by doing:

> var tokenContract = eth.contract([{
     "type":"function",
     "name":"balanceOf",
     "constant":true,
     "payable":false,
     "inputs":[{"name":"","type":"address"}],
     "outputs":[{"name":"","type":"uint256","value":"0"}]
}]);

And then you can do:

> var erc20ContractAddress = "<ADSRESS_OF_TOKEN'S_CONTRACT>";
> var account = "YOUR_ADDRESS";
> tokenContract.at(erc20ContractAddress).balanceOf(account);

This outputs the token balance in plain tokens, i.e. without showing a decimal point.