A Beginner's Guide to Interacting with Ethereum Smart Contracts Using MetaMask and EthJS

Ethereum smart contracts are self-executing contracts with the terms of the agreement directly written into code. To interact with these contracts from a web application, developers need specialized tools and libraries. This guide explores how to use MetaMask, web3, and EthJS to call and interact with smart contracts on the Ethereum blockchain.

Understanding the Tools: MetaMask, web3, and EthJS

MetaMask is a popular cryptocurrency wallet and browser extension that allows users to manage their Ethereum accounts and interact with decentralized applications (dApps) directly from their browsers. It injects a web3 provider into the web page, enabling seamless communication with the Ethereum network.

Web3.js is the original JavaScript library for Ethereum, providing a comprehensive set of methods to interact with the blockchain. However, it can be quite heavy for some applications.

EthJS is a lightweight alternative to web3.js, specifically designed for efficiency and optimization. According to its official documentation, EthJS is "a highly optimised, light-weight JS utility for Ethereum based on web3.js, but lighter, async only and using BN.js."

Setting Up EthJS for Contract Interaction

To begin using EthJS for smart contract interactions, you'll need to install the necessary modules:

npm install ethjs-query ethjs-contract --save

These modules provide the query functionality and contract interaction capabilities needed to work with Ethereum smart contracts.

Initializing EthJS with MetaMask's Provider

The first step in setting up your application is to initialize EthJS using MetaMask's web3 provider:

var Eth = require('ethjs-query');
var EthContract = require('ethjs-contract');

if (typeof web3 !== 'undefined') {
  eth = new Eth(web3.currentProvider);
  contract = new EthContract(eth);
  startApp();
} else {
  alert("No currentProvider for web3");
}

This code checks if web3 is available (indicating MetaMask is installed) and then creates instances of EthJS Query and EthJS Contract using the provider from MetaMask.

Configuring Your Contract Instance

To interact with a specific smart contract, you need two crucial pieces of information: the contract's ABI (Application Binary Interface) and its deployed address:

function startApp() {
  const abi = [
    {
      "constant": true,
      "inputs": [],
      "name": "data",
      "outputs": [
        {
          "name": "",
          "type": "uint256"
        }
      ],
      "payable": false,
      "stateMutability": "view",
      "type": "function"
    },
    {
      "anonymous": false,
      "inputs": [
        {
          "indexed": true,
          "name": "_from",
          "type": "address"
        },
        {
          "indexed": false,
          "name": "value",
          "type": "uint256"
        }
      ],
      "name": "Set",
      "type": "event"
    },
    {
      "constant": false,
      "inputs": [
        {
          "name": "x",
          "type": "uint256"
        }
      ],
      "name": "set",
      "outputs": [],
      "payable": false,
      "stateMutability": "nonpayable",
      "type": "function"
    }
  ];
  
  const address = '0x06e1c13546e04514a0cf8d842216a84745ac317a';
  const SimpleStorage = contract(abi);
  const simpleStorage = SimpleStorage.at(address);
  
  listenForClicks(simpleStorage);
}

The ABI defines how to interact with the contract, while the address specifies where the contract is deployed on the blockchain.

Creating a User Interface for Contract Interaction

A simple HTML interface allows users to interact with the smart contract:

<input type="number" id="data-value" placeholder="Enter a number">
<button class="set">Set Data!</button>

This interface includes an input field for entering data and a button to trigger the contract interaction.

Implementing the Contract Interaction Logic

The JavaScript function that handles the contract interaction looks like this:

function listenForClicks(simpleStorage) {
  var button = document.querySelector('button.set');
  
  button.addEventListener('click', function() {
    var value = document.getElementById('data-value').value;
    
    simpleStorage.set(value, {
      from: "0x123abc000..."
    }, function(error, result) {
      if (error) {
        console.debug(error);
        return;
      }
      // Returns transaction hash as result
      console.debug(result);
    });
  });
}

This code listens for clicks on the button, retrieves the value from the input field, and calls the set function on the smart contract. One of EthJS's advantages is that you don't need to manually construct transactions or calculate function signatures—it handles these complexities for you.

👉 Explore advanced blockchain development tools

Step-by-Step Interaction Process

Enter any number in the input field and click "Set Data!"
MetaMask will display a confirmation dialog asking the user to authorize the transaction
After clicking confirm, the transaction is broadcast to the Ethereum network
Once the transaction is successful, you'll receive a transaction hash
After the transaction is confirmed, you can call the contract's data() function to verify the updated value

Reading Data from the Contract

To retrieve data from the contract, you can use EthJS's simple calling mechanism:

simpleStorage.data(function(error, result) {
  // result[0] is a BN.js object
  console.debug(result[0].toNumber());
});

// Expected output: 20 (or whatever value was set)

This function call doesn't require a transaction since it's only reading data from the contract, not modifying the blockchain state.

Best Practices for Ethereum dApp Development

When developing dApps that interact with smart contracts, consider these best practices:

Always validate user input before sending transactions
Handle errors gracefully and provide user-friendly feedback
Consider gas costs and network congestion when designing interactions
Implement proper loading states while transactions are processing
Test thoroughly on test networks before deploying to mainnet

Frequently Asked Questions

What is the difference between web3.js and EthJS?
Web3.js is the original Ethereum JavaScript API with comprehensive features, while EthJS is a lighter, more optimized alternative focused on async operations. EthJS is particularly beneficial for applications where bundle size is a concern.

Do I need MetaMask to interact with Ethereum smart contracts?
While MetaMask is not strictly required, it significantly simplifies the process by handling account management and transaction signing. Alternative options include other web3 providers or directly connecting to Ethereum nodes.

How do I handle different types of smart contract functions?
View functions (that only read data) can be called directly without transactions, while state-changing functions require transactions and gas fees. EthJS provides a uniform interface for both types of interactions.

What should I do if a transaction fails?
Always implement error handling in your callbacks. Common reasons for failure include insufficient gas, rejected transactions, or network issues. Provide clear feedback to users about what went wrong.

How can I estimate gas costs before sending transactions?
While EthJS simplifies transaction sending, you can still estimate gas costs using the underlying provider methods. This helps set appropriate gas limits and avoid failed transactions.

Can I use EthJS with other Ethereum-compatible blockchains?
Yes, EthJS can work with any Ethereum Virtual Machine (EVM) compatible blockchain by connecting to the appropriate provider for that network.

Developing applications that interact with Ethereum smart contracts requires understanding both the blockchain fundamentals and the tools available for seamless integration. By leveraging MetaMask's provider system with EthJS's optimized library, developers can create efficient and user-friendly decentralized applications that bridge the gap between traditional web interfaces and blockchain functionality.