Returning a value from an external function

Question

I have in a contract:

    function createCourt() external returns (uint256) {
        uint256 _id = ++nonce;
        courtOwners[_id] = msg.sender;
        return _id;
    }

Now, when I try to call this function from web3.js I stumble an impossibility to return values.

I was recommened to use events instead to return a value. But if I and another user calls createCourt():

    function createCourt() external {
        uint256 _id = ++nonce;
        courtOwners[_id] = msg.sender;
        emit CourtCreated(_id);
    }

how can I determine which of several events returned my court, not another one?

Or any other way to get the court number after I create it?

Answer 1

You use the transaction's hash to obtain the transaction's receipt, if you made a single call to createCourt you have a single event in the receipt.

Another option is to accept an random input parameter and use that as id.

Answer 2

You can add second parameter to your event:

event CourtCreated(address indexed _address, uint256 _id);

And later on execute the event in your methods like this:

emit CourtCreated(msg.sender, _id);

Now with the keyword indexed before _address parameter with web3.js you can filter the events by the address. You can check which events are executed by you and which by other users.

If you are using web3 0.2.x you have to use method ContractInstance.myEvent() - read more here or if you are using web3 1.x.x you have to use ContractInstance.getPastEvents() - read more here.