How to find out the value using call() method in truffle and testrpc?

Question

       pragma solidity ^0.4.2;

       contract Transfer {

                address public owner;  
                mapping (address => uint) balances;

               // constructor
               function Transfer() {
                             owner=msg.sender;
                             balances[owner]=1000;
               }

               function transferToUser(address _to, uint _value) returns (bool success)  {

                 if(balances[msg.sender]< _value){
                     return false;
                  }

                  balances[msg.sender] -= _value;
                  balances[_to] += _value;
                  return true;
           }
          function getBalance(address _user) returns (uint _balance){
                  return balances[_user];
          }

     }

This is the code for contract i have deployed using testrpc and truffle.

I have used the following commands in truffle.

      var contract=Transfer.deployed();
      contract.owner.call().then(console.log);

The owner is printed correctly.Then used call() method to print balance.

      contract.balances[owner].then(console.log);

This call method returned error.How can we print the value in balances[owner]?

The functions defined in contract is called as follows.

          contract.transferToUser("0x4f91a3661a18328bc5d995a1f8b63cc69778a529",300).then(console.log)


         contract.getBalance("0x4f91a3661a18328bc5d995a1f8b63cc69778a529").then(console.log)

Both functions returned hash value of transaction.How can we check the balances returned from the function getBalance() such as balances[msg.sender],balances[_to],balances[_user]?

Answer 1

You've got the right idea with checking balances for one address at a time and using promises.

The square brackets are weird here. Looks like you're adapting array style but this is a function call that returns one number. You got a txhash because you sent a transaction to be mined ... different than call() which is local, read only, faster, free (no gas) and gives the returned values.

contract.balances[owner].then(console.log);

Try

contract.balances.call(owner).then(console.log);

You're pretty close here. I just built it out a bit.

contract.owner.call()
.(then(function(ownerReturned) {
  console.log("got owner", ownerReturned);
})
.catch(function(err) {
 console.error("problem getting owner", err;
});

and, with round brackets ...

contract.balance.call(owner)
.then(function(balanceReturned) {
  // balanceReturned will be in bigNumber format
  // try ...
  var bal = balanceReturned.toString(10);
  console.log("Balance returned", bal);   
})
.catch(function(error) {
  console.error("Error getting balance", error):
})

With luck, I haven't given you something with errors in it. It's a bit of scribble on my part.

For the transfer function, it needs to update the state, so drop call() like this, get a receipt and wait for it to be mined (not shown):

transferToUser(args, args).
.then(function(txn) {
  // Add something to wait for it to be mined.
  // after it's mined, this should work
  return contract.balance.call(args);
})
  .then(function(returnedBalance) {
  // carry on with the updated balance in hand
});

Hope it helps.