In this tutorial, you’ll build a Node.js application that connects to the IoTeX testnet using Web3.js and interacts with the smart contract you just deployed inDeploy a simple contract.

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Step 1: Set Up Your Project

mkdir iotex-client
cd iotex-client
npm init -y
npm install web3

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Step 2: Connect to the IoTeX Testnet

Create a file index.js and initialize Web3: 
const { Web3 } = require('web3');

const web3 = new Web3('https://babel-api.testnet.iotex.io');

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Step 3: Prepare Contract Details

Replace the following placeholders with your actual contract address from Deploy a simple contract. 
const contractAddress = 'YOUR_DEPLOYED_CONTRACT_ADDRESS';
const abi = [
  {
    "anonymous": false,
    "inputs": [
      { "indexed": false, "internalType": "address", "name": "sender", "type": "address" },
      { "indexed": false, "internalType": "string", "name": "message", "type": "string" }
    ],
    "name": "MessagePosted",
    "type": "event"
  },
  {
    "inputs": [
      { "internalType": "string", "name": "message", "type": "string" }
    ],
    "name": "postMessage",
    "outputs": [],
    "stateMutability": "nonpayable",
    "type": "function"
  }
];

const contract = new web3.eth.Contract(abi, contractAddress);

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Step 4: Send a Message

Continue editing index.js with the following and replace YOUR_PRIVATE_KEY with your Metamask testnet private key: 
// Your Metamask testnet private key:
// Must include the 0x prefix when using Web3.js in JavaScript
const privateKey = '0xYOUR_PRIVATE_KEY'; 

const account = web3.eth.accounts.privateKeyToAccount(privateKey);
web3.eth.accounts.wallet.add(account);

async function sendMessage(text) {
  console.log(`Sending message "${text}" to the IoTeX testnet...`);

  const tx = contract.methods.postMessage(text);
  const gas = await tx.estimateGas({ from: account.address });

  const receipt = await tx.send({
    from: account.address,
    gas
  });

  console.log('Transaction receipt:', receipt);
}

sendMessage("Hello from IoTeX!");
To test your interaction script, simply run: 
node index.js
This will send your message to the smart contract and print the transaction receipt. You’re interacting with smart contracts on IoTeX Layer 1!
What’s next?