[Ethereum] the correct way to access public variable from contract in Truffle: then or standard callback

truffleweb3js

There are some similar posts to this but none seem to be able to solve my problem.

I have a contract deployed with Truffle running with testrpc.

I want to access the value of a public uint variable named gameStatus defined in the deployed contract game variable as such:

uint public gameStatus = 23;

I have found two ways to do this:

game.gameStatus(function(err, res){
    document.getElementById('amt').innerText = res;
});

And using the then operator from Promises.

game.gameStatus().then(function(result){
    document.getElementById('tableamt').innerText = result;
});

The first method never affects the value of 'tableamt' in the HTML. And the second method always yields a value of '0' in the HTML.

The contract in both cases deploys without errors and web3 does not complain.

Any help is much appreciated.

Thanks!

Best Answer

To get public variable value you need to use call. It is evaluated directly on local node without sending transaction to the blockchain. See also this great answer.

So, you need to call it that way:

game.gameStatus.call(function(err, res){
    document.getElementById('amt').innerText = res;
});

Related Solutions

[Ethereum] Iterating through an array in JS and simple promises (web3/pudding)

The problem is the result of using a Javascript Promise (provided by Pudding) that wraps the call. If you made the same call without a Promise wrapper it would work as you expect. But in your case the Promise is not resolving until you hit the end of the loop.

Truffle Solidity – Solving Problem with Console: Cannot Read Property ‘Call’ of Undefined

If you are running the latest version of Truffle (v.3.x), the way to call a function or variable in your contract has evolved a little bit according to the Upgrading documentation

In Truffle 2.0, your contract abstractions managed your networks in a naive way, and added constructs like a "default network" that opened up the possibility of using the wrong network artifacts and deployed addresses at the wrong time. This provided a fancy and easy to use syntax – i.e., MyContract.deployed().myFunction(...) – but it left your code open to errors. Truffle 3.0 changes this syntax, where .deployed() is now thennable, like a promise (see example below). Similarly, this makes your contract abstractions seamlessly integrate with EIP 190, Ethereum's package management standard. All of this is for the better, but it means you'll have some changes to make all across the board.

So in your case, if you want to call a function, you have to do this:

HelloWorld.deployed().then(function(instance) {
    return instance.balance.call();
}).then(function(balance) {
    console.log(balance);
});

Please refer to the latest documentation for more details.

Best Answer

Related Solutions

[Ethereum] Iterating through an array in JS and simple promises (web3/pudding)

Truffle Solidity – Solving Problem with Console: Cannot Read Property ‘Call’ of Undefined

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