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Call a contract

This tutorial uses Web3.js to execute a function in a smart contract. Calling the contract function is a type of transaction that requires paying gas.

Prerequisites​

Before attempting this tutorial, ensure you completed the tutorial to deploy a contract and updated the .env file with the smart contract address.

Steps​

1. Create a script to execute the contract​

In the deployContract folder created earlier, create the JavaScript file that executes a contract function and signs the transaction.

In this example we'll create the call.js file with the following contents:

const { Web3 } = require("web3");

// Loading the contract ABI
// (the results of a previous compilation step)
const fs = require("fs");
const { abi } = JSON.parse(fs.readFileSync("Demo.json"));

async function main() {
  // Configuring the connection to an Ethereum node
  const network = process.env.ETHEREUM_NETWORK;
  const web3 = new Web3(
    new Web3.providers.HttpProvider(
      `https://${network}.infura.io/v3/${process.env.INFURA_API_KEY}`,
    ),
  );
  // Creating a signing account from a private key
  const signer = web3.eth.accounts.privateKeyToAccount(
    "0x" + process.env.SIGNER_PRIVATE_KEY
  );
  web3.eth.accounts.wallet.add(signer);
  // Creating a Contract instance
  const contract = new web3.eth.Contract(
    abi,
    // Replace this with the address of your deployed contract
    process.env.DEMO_CONTRACT,
  );
  // Issuing a transaction that calls the `echo` method
  const method_abi = contract.methods.echo("Hello, world!").encodeABI();
  const tx = {
    from: signer.address,
    to: contract.options.address,
    data: method_abi,
    value: '0',
    gasPrice: '100000000000',
  };
  const gas_estimate = await web3.eth.estimateGas(tx);
  tx.gas = gas_estimate;
  const signedTx = await web3.eth.accounts.signTransaction(tx, signer.privateKey);
  console.log("Raw transaction data: " + ( signedTx).rawTransaction);
  // Sending the transaction to the network
  const receipt = await web3.eth
    .sendSignedTransaction(signedTx.rawTransaction)
    .once("transactionHash", (txhash) => {
      console.log(`Mining transaction ...`);
      console.log(`https://${network}.etherscan.io/tx/${txhash}`);
    });
  // The transaction is now on chain!
  console.log(`Mined in block ${receipt.blockNumber}`);
}

require("dotenv").config();
main();

2. Call the contract​

Run the script to call the smart contract:

node call.js
info

If you receive an insufficient funds for gas error, then you may need to top up your signer account with some testnet ETH.

The script will display the details of the transaction in the output. View the transaction in a block explorer such as Etherscan.