How to unmarshall an uint returned from a solidity contract?

Question

I've compiled and deployed the following trivial smart contract:

pragma solidity ^0.4.7;    
/// Trivial counter contract. Provides an external function which increments the counter
/// and replies the current value.
contract BasicCounter {
    uint private counter = 0;
    function incrementAndGet() external returns (uint) {
        counter++;
        return counter;
    }
}

In the node REPL I can instantiate a handle for the deployed smart contract:

let instance = 
    web3.eth.contract(/* the contract abi */))
        .at(/* the contract address  */);

Using the instance, I can invoke the external function incrementAndGet:

let ret = instance.incrementAndGet();

The return value ret is a hex string, i.e.

0xa768b61d2892dda6495b6b1529e4c81252f333f4ca4ffa18ea7408580366292d

If I convert it to a BigNumberusing web3, i.e.

let value = web3.toBigNumber(ret);
console.log(`: <${value.toString(10)}>`)

I get the following value

current counter value: <75721254712088916818789556586747754611065627738232115122133311013811340585261>

This looks strange. The first invocation of incrementAndGet should rather return something like 00000......00001.

Any hint what is going wrong here? How to correctly unmarshal an uint value returned from a external function in a smart contract?

Answer 1

The issue is that you can't return a value from a transaction. You can use

let ret = instance.incrementAndGet.call();

to see view the current counter value.

See What is the difference between a transaction and a call?