Best way for web3 to receive data from function return?

Question

Now I'm trying to delelop my first Dapp (using Metamask plugin). everything is going to be all right, but I can't get one thing: Is it possible to receive return from function, using Web3 (not JS VM!)

For example, I have a simple contract like this:

pragma solidity ^0.4.0;

contract test {
    int32 data = 123;

    function getData() returns (int32) {
        return data;
    }
}

When I'm calling getData() with JS virtual machine, it works fine:

But with selected Inject web3 (Metamask) it returns information about new transaction, instead of 123

I also can't receive any data from the JS console (Metamask):

The last example in the JS console looks like it should work. Maybe this question needs to focus on that part. — eth, Apr 05 '17 at 23:43
@eth Yep, it works fine, but I get txn hash instead of 123, as expected — Sergey Potekhin, Apr 05 '17 at 23:47
Sorry, I misread the console output as error being undefined and data as null. The txn hash is returned because a transaction was issued by web3 instead of a "call" (hope you saw the question that asks about the difference). — eth, Apr 06 '17 at 04:28

score 4 · Accepted Answer · edited Apr 13 '17 at 13:01

4

For the question, the simplest is to make getData constant as:

function getData() constant returns (int32)

Another option is to use an event: How to get return values when function with argument is called?

edited Apr 13 '17 at 13:01

Community

1

answered Apr 05 '17 at 23:35

eth

85,679
53
285
406

Adding constant modifier solved all, thanks a lot! – Sergey Potekhin Apr 06 '17 at 01:08

score 3 · Answer 2 · answered Apr 06 '17 at 01:18

3

Did you try using the call function asynchronously:

contract.getData.call().then(function(result) {
  console.log(result);
});

answered Apr 06 '17 at 01:18

TyndieRock

171
6

I've had pretty good success using Promise.promisifyAll (after contract instantiation) so you can use actual return values (Promises) instead of callbacks, if return values are important to you. – Paul S Apr 06 '17 at 07:30

Best way for web3 to receive data from function return?

2 Answers2