JavaScript Console 2: Contracts

Last edited on August 14, 2023

The Introduction to the Javascript console page outlined how a Javascript console can be attached to Geth to provide a more user-friendly interface to Ethereum than interacting directly with the JSON-RPC API. This page will describe how to deploy contracts and interact with contracts using the attached console. This page will assume the Javascript console is attached to a running Geth instance using IPC. Clef should be used to manage accounts.

Deploying a contract

First we need a contract to deploy. We can use the well-known Storage.sol contract written in Solidity. The following Solidity code can be copied and pasted into a text editor and saved as go-ethereum/storage-contract/Storage.sol.

// SPDX License-Identifier: GPL 3.0

pragma solidity ^0.8.0;

contract Storage{

    uint256 value = 5;

    function set(uint256 number) public{
        value = number;
    }

    function retrieve() public view returns (uint256){
        return value;
    }
}

The contract needs to be compiled before Geth can understand it. Compiling the contract creates an Application Binary Interface and the contract bytecode. This requires a Solidity compiler (e.g. solc) to be installed on the local machine. Then, compile and save the ABI and bytecode to a new build subdirectory using the following terminal commands:

cd ~/go-ethereum/storage-contract
solc --bin Storage.sol -o build
solc --abi Storage.sol -o build

The outputs look as follows:

# build/Storage.bin
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

# Storage.abi
[{"inputs":[],"name":"retrieve","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"stateMutability":"view","type":"function"},{"inputs":[{"internalType":"uint256","name":"number","type":"uint256"}],"name":"store","outputs":[],"stateMutability":"nonpayable","type":"function"}]

These are all the data required to deploy the contract using the Geth Javascript console. Open the Javascript console using ./geth attach geth.ipc.

Now, for convenience we can store the abi and bytecode in variables in the console:

var abi = [
  {
    inputs: [],
    name: 'retrieve',
    outputs: [{ internalType: 'uint256', name: '', type: 'uint256' }],
    stateMutability: 'view',
    type: 'function'
  },
  {
    inputs: [{ internalType: 'uint256', name: 'number', type: 'uint256' }],
    name: 'store',
    outputs: [],
    stateMutability: 'nonpayable',
    type: 'function'
  }
];

var bytecode =
  '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';

The ABI can be used to create an instance of the contract:

var contract = eth.contract(abi);

This contract instance can then be deployed to the blockchain. This is done using eth.sendTransaction, passing the contract bytecode in the data field. For convenience we can create a transaction JSON object first, then pass it to eth.sendTransaction later. Let's use the first account in eth.accounts as the sender. The amount of gas to include can be determined using eth.estimateGas:

var gas = eth.estimateGas({ data: bytecode });

Note that each command that touches accounts will require approval in Clef unless a custom rule has been implemented. The bytecode, gas and address of the sender can be bundled together into an object that will be passed to the contract's new() method which deploys the contract.

var tx = { from: eth.accounts[0], data: bytecode, gas: gas };
var deployed_contract = contract.new(tx);

The transaction hash and deployment address can now been viewed in the console by entering the variable name (in this case deployed_contract):

{
  abi:[{
    inputs: [],
    name: "retrieve",
    outputs: [{...}],
    stateMutability: "view",
    type: "function"
  },{
    inputs: [],
    name: "store",
    outputs: [{...}],
    stateMutability: "nonpayable",
    type: "function"
  }],
  address: "0x2d6505f8b1130a22a5998cd31788bf6c751247f",
  transactionHash: "0x5040a8916b23b76696ea9eba5b072546e1112cc481995219081fc86f5b911bf3",
  allEvents: function bound(),
  retrieve: function bound(),
  store: function bound()
}

Passing the transaction hash to eth.getTransaction() returns the deployment transaction details including the contract bytecode in the input field. To interact with the contract, create an instance by passing the deployment address to contract.at then call the methods.

var instance = contract.at('0x2d6505f8b1130a22a5998cd31788bf6c751247f');
// store() alters the state and therefore requires sendTransaction()
contract.set.sendTransaction(42, { from: eth.accounts[0], gas: 1000000 });
// retrieve does not alter state so it can be executed using call()
contract.retrieve().call() >> 2;

Summary

This page demonstrated how to create, compile, deploy and interact with an Ethereum smart contract using Geth's Javascript console.