Web3.js can access coinbase but not use it in transactions

Question

This question is about web3.js

I'm working on a very simple dapp to test out making a token. There's only two functions in my contract - one to mint coins and one to transfer them. I'm trying to make a fountain site where people can go get my token

The part that's confusing me is how I'm accessing the visitors account. This code snippet works - it'll let me grab their coinbase and use it as their default address.

var contractAddress = '0x791E39A210B49811459531A54790377540eFfcde';
var myCoin = eth.contract(contractABI).at(contractAddress);
var wallet = web3.eth.defaultAccount = web3.eth.coinbase;

var transaction = myCoin.mint(wallet, value)

But if I change it to not set the default address, i get an invalid address error. Below code does NOT work, despite me being able to log web3.eth.coinbase in the console no problem.

var contractAddress = '0x791E39A210B49811459531A54790377540eFfcde';
var myCoin = eth.contract(contractABI).at(contractAddress);
var wallet = web3.eth.coinbase;

var transaction = myCoin.mint(wallet, value)

Can anyone explain this implementation choice in web3.js? Why is my contract method dependent on defaultAccount in this way?

Answer 1

Every transaction requires a sender, a from account that pays for the gas. An invalid address error usually means that the sender hasn't been specified.

You can explicitly pass from for every transaction, like:

myCoin.mint(wallet, value, {from: web3.eth.coinbase})

Or specify it just once using web3.eth.defaultAccount = web3.eth.coinbase. The defaultAccount is then used whenever the from is not explicitly provided.