eth-lightwallet: what is the difference between a functionTx and a valueTx?

Question

In eth-lightwallet, there are two types of Tx creation functions;

txutils.valueTx(txObject)

and

txutils.functionTx(abi, functionName, args, txObject)

My guess is; although their inputs are very different, they both result in a Tx object that looks like this:

tx = {
  nonce: '..',
  gasPrice: '..', 
  gasLimit: '..',
  to: '..', 
  value: '..', 
  data: '...'
}

and the only difference is that data is empty in valueTx, and it's the hex-encoded version of the function call in functionTx. Is this correct? Is data the only difference between two?

Also, how can I compute data myself, given that I know the contract address, abi and the function name/parameters? Is there another library for this?

Thanks,

Answer 1

The difference between the two is that valueTx simply creates a transaction that sends a 'value' (ether/wei etc.) to a certain address on the blockchain, as opposed to functionTx that creates a transaction that executes a function on the blockchain (note that you can also pass a 'value' in this case along with the function call).

The 'data' argument is a hex representation of the arguments that a function receives, and there are a few different methods to create it.

I'm not sure how you use the library, but if you use web3, the easiest way in my opinion is to generate the data by calling getData on the function object.

For example, if you want to create the hex data that represents the arguments that you want to pass to a 'foo' function, you can do the following -

// for a given foo function in the contract -
// function foo(uint256 a, uint256 b) {}

// create a contract instance -
var contract = web3.eth.contract(contractAbi);

// generate the hex data for the arguments
var data = contract.foo.getData(100, 200);

// at this point, data value will be the hex string that
// represents the "100,  200" arguments.
// note that you might need to add the '0x' prefix to
// the data if it's not already there, some wallets are strict about it