How to get the return value from a smart contract method execution

Question

I have ERC-20 token smart-contract which methods I call using sendSignedTransaction from web3.js. After I know transaction is succesfully mined I need to check contract method execution result. How do I do it if all I have is transaction hash?

Example: method transferFrom(from, to, tokens) returns true or false depending on whether transferring was successful. So if I try to transfer 100 tokens from empty wallet, contract method will return false.

As I understand, there is no way of determining method outcome using txHash after transaction is mined and confirmed. Then which ways exists to handle this case? How can I make sure that tokens were transferred?

Answer 1

You can emit an event inside your contract code ( actually in ERC20 standard there is always a transfer event present ) and then inside web3js, read all the events till latest block using this line of code:

Events = Contract.eventName({}, {fromBlock: 0, toBlock: 'latest'});