How to get the return value on a state altering function?

Question

I'm playing around with ERC721 tokens.

I have this mint function in my contract that is supposed to return a value, but in my client app (using Web3), the result I get from this function call only contains the transaction id.

Here is the function :

    function mint() public returns (uint) {
        uint newId = currentTokenId + 1;
        _mint(msg.sender, newId);
        currentTokenId = newId;
        return currentTokenId;
    }

The point of this is just making an interface to issue/mint new tokens out of this smart contract. Here is how I call it in my client app :

this.contract.methods.mint().send({from: this.account}, function(error, result) {
    console.log(result) // 0x08e...00d0
})

Am I missing something? Am I misusing the Web3 framework? Or maybe I just misunderstood the return statement in Solidity?

Answer 1

Turns out I can only get the transaction hash for the _mint call, but I was able to recover the data (currentTokenId) by checking the transaction event by it's hash.

I'm not sure it's the best solution but here is how I've done it :

this.mint = function(callback) {
    var mintTxHash = null
    this.contract.methods.mint().send({from: this.account}, function(error, result) {
        mintTxHash = result
    })
    this.contract.events.allEvents((error, event) => {
        if (event != null && mintTxHash != null) {
            if (event.transactionHash == mintTxHash) {
                callback(event.returnValues.tokenId)
            }
        }
    })
}

Not sure it will help anyone but ¯_(ツ)_/¯ If you have a better solution, please don't hesitate to post!