web3 calls Solidity contract function with extra parameter?

Question

In the http://truffleframework.com/tutorials/pet-shop example, there is the following contract, and the function adopt(uint petId) has only one parameter.

contract Adoption {
    address[16] public adopters;

    function adopt(uint petId) public returns (uint) {
        require(petId >= 0 && petId <= 15);
        adopters[petId] = msg.sender;
        return petId;
    }

    function getAdopters() public returns (address[16]) {
        return adopters;
    }
}

However, in the javascript code app.js , the handleAdopt function call the contract function using the following code.

  App.contracts.Adoption.deployed().then(function(instance) {
    adoptionInstance = instance;
    return adoptionInstance.adopt(petId, {from: account});
  })

The function is called with the extra object {from: account} . Why? And is this parameter discarded in the solidity code?

BTW, there is an undefined global variable web3 ? Is the value be assigned by the MetaMask extension?

Answer 1

That is the transactionObject which describes general information about all transaction calls (gas limit, price, amount of ether to send, etc.). The JS code you posted is using the web3 library. That's not the direct call to the contract API. The web3 library converts it to an RPC. The transactionObject comes after all of the contract parameters. There is another parameter that comes after which is the callback with the results of the contract call (see here ).

These are all of the options for the transactionobject described from the docs :

• from: String - The address for the sending account. Uses the web3.eth.defaultAccount property, if not specified.
• to: String - (optional) The destination address of the message, left undefined for a contract-creation transaction.
• value: Number|String|BigNumber - (optional) The value transferred for the transaction in Wei, also the endowment if it's a contract-creation transaction.
• gas: Number|String|BigNumber - (optional, default: To-Be-Determined) The amount of gas to use for the transaction (unused gas is refunded).
• gasPrice: Number|String|BigNumber - (optional, default: To-Be-Determined) The price of gas for this transaction in wei, defaults to the mean network gas price.
• data: String - (optional) Either a byte string containing the associated data of the message, or in the case of a contract-creation transaction, the initialisation code.
• nonce: Number - (optional) Integer of a nonce. This allows to overwrite your own pending transactions that use the same nonce.