All discussions on this mention that it's impossible to get a returned value from sendTransaction() run on a contract function, where the contract state is being changed. I don't understand why the returned value can't be recorded in the transaction log on the blockchain, similarly to events, and so then it could be retrieved on the transaction confirmation:
web3.eth.sendTransaction(...)
.on('confirmation', function(1, receipt){ ... // retrieving value returned by smart contract function here })
Logs are made for describing the events emitted from the contract - which is the current solution for getting data from transactions - , so the return data can't go in there.
Including a return_data in the receipt, though, has been discussed and apparently forgotten. EIP758, has the following sollution:
EIP 658 originally proposed adding return data to transaction receipts. However, return data is not charged for (as it is not stored on the blockchain), so adding it to transaction receipts could result in DoS and spam opportunities. Instead, a simple Boolean
statusfield was added to transaction receipts. This modified version of EIP 658 was included in the Byzantium hard fork. While thestatusfield is useful, applications often need the return data as well.
The primary advantage of using the strategy outlined here is efficiency: no extra data needs to be stored on the blockchain, and minimal extra computational load is imposed on nodes. Since light clients have the current state, they can compute and send return data notifications without contacting a server. Although after-the-fact lookups of the return value would not be supported, this is consistent with the conventional use of return data, which are only accessible to the caller when the function returns, and are not stored for later use.
And this go client pull request, which didn't go through because the best solution would be, instead, an ethereum hard fork - even though we had some since then and it didn't happen.