Advanced tutorial

Create a program to interface with the blockchain and change its value

Install Web3.js using npm by running the following command:

npm install web3

Create a new JavaScript file (e.g., interact.js) and paste the following code into it. Replace with an account from your Ganache instance, with your contract's address, and with the ABI from the build/contracts/MyContract.json file:

A Contract Application Binary Interface (ABI) is a JSON representation of a smart contract's functions and events written in the Solidity programming language for Ethereum blockchain. ABI acts as an interface between the Ethereum blockchain and the outside world, allowing users and other contracts to understand and interact with the smart contract.

The ABI contains information such as: Function names Function input and output parameter types Function modifiers (e.g., public, private, view, or payable) Event names and parameters Event indexed attributes

By having a standardized format, the ABI enables developers and applications to encode and decode data for transactions and contract interactions properly. When using popular tools like Web3.js or Ether.js, the ABI is typically required to create an instance of a smart contract, allowing developers to call its functions, listen to its events, and interact with it programmatically.

When a Solidity contract is compiled, the compiler generates the ABI alongside the bytecode, which is then deployed to the Ethereum blockchain. Developers usually store and share the ABI alongside the contract's address to facilitate interactions with the contract.

const Web3 = require('web3');

// Replace with your Ganache instance URL
const providerURL = 'http://localhost:8545';

// Replace with your Ganache account
const account = '';

// Replace with your contract address and ABI
const contractAddress = '';
const Web3 = require('web3');

// Replace with your Ganache instance URL
const providerURL = 'http://localhost:8545';

const contractABI = JSON.parse('[{"inputs":[],"name":"value","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"stateMutability":"view","type":"function","constant":true},{"inputs":[{"internalType":"uint256","name":"newValue","type":"uint256"}],"name":"setValue","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[],"name":"getValue","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"stateMutability":"view","type":"function","constant":true}]');

async function main() {
  const web3 = new Web3(providerURL);
  const contract = new web3.eth.Contract(contractABI, contractAddress);

  // Call the setValue function
  const newValue = 45;
  const setValueTx = contract.methods.setValue(newValue);
  const gas = await setValueTx.estimateGas({ from: account });
  const receipt = await setValueTx.send({ from: account, gas });
  console.log('setValue transaction receipt:', receipt);

  // Call the getValue function
  const value = await contract.methods.getValue().call();
  console.log('Value from contract:', value);
}

main().catch(console.error);
  

Run the script using Node.js:

node interact.js
  

In the provided code snippet, we don't need the private key directly, as the Ganache instance is configured to automatically sign the transactions for the accounts it manages. When using Web3.js with Ganache, you can interact with the smart contract using only the account's Ethereum address. However, if you were to use this code snippet with a real Ethereum network, such as Mainnet or a testnet like Rinkeby, you would need to sign the transactions with the private key associated with the Ethereum address. In this case, you could use Web3.js or Ethers.js to handle the signing process. Here's an example of how to sign transactions using Web3.js with a private key: First, install the ethereumjs-tx package to handle the transaction signing:

npm install ethereumjs-tx

Modify the interact.js file to include the private key and use ethereumjs-tx to sign the transactions. Replace with the private key of the Ethereum account you're using:

const Web3 = require('web3');
const EthereumTx = require('ethereumjs-tx').Transaction;

// Replace with your Ganache instance URL
const providerURL = 'http://localhost:8545';

// Replace with your Ganache account and private key
const account = '';
const privateKey = '';
const privateKeyBuffer = Buffer.from(privateKey.slice(2), 'hex');
// Replace with your contract address and ABI
const contractAddress = '';
const contractABI = JSON.parse('[{"inputs":[],"name":"value","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"stateMutability":"view","type":"function","constant":true},{"inputs":[{"internalType":"uint256","name":"newValue","type":"uint256"}],"name":"setValue","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[],"name":"getValue","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"stateMutability":"view","type":"function","constant":true}]');

async function main() {
  const web3 = new Web3(providerURL);
  const contract = new web3.eth.Contract(contractABI, contractAddress);

  // Call the setValue function
  const newValue = 42;
  const setValueTx = contract.methods.setValue(newValue);
  const data = setValueTx.encodeABI();
  const nonce = await web3.eth.getTransactionCount(account);
  const gas = await setValueTx.estimateGas({ from: account });

  const txParams = {
    nonce: web3.utils.toHex(nonce),
    gasPrice: web3.utils.toHex(web3.utils.toWei('10', 'gwei')),
    gasLimit: web3.utils.toHex(gas),
    to: contractAddress,
    data: data
  };

  const tx = new EthereumTx(txParams, { chain: 'mainnet' });
  tx.sign(privateKeyBuffer);

  const serializedTx = tx.serialize();
  const rawTx = '0x' + serializedTx.toString('hex');

  const receipt = await web3.eth.sendSignedTransaction(rawTx);
  console.log('setValue transaction receipt:', receipt);

  // Call the getValue function
  const value = await contract.methods.getValue().call();
  console.log('Value from contract:', value);
}

main().catch(console.error);