Update Token Balance After Transaction Ethers.js

Aug 20, 2025 by Pedro Alvarez 49 views

How to Get the Latest Token Balance After a Transaction in Ethers.js

Hey guys! Diving into the world of dApp development with Ethers.js is super exciting, especially when you're building something cool like a Uniswap clone. It’s awesome that you’re tackling this, and it’s totally normal to hit a few bumps along the way, like figuring out how to update token balances after a transaction. No worries, we’ve all been there! Let’s break down how you can keep those balances fresh and accurate in your dApp.

Understanding the Challenge

First off, it’s important to understand why this isn’t as straightforward as you might think. When you make a transaction on the blockchain, the change isn't instantly reflected everywhere. Blockchains are decentralized, which means updates need to be confirmed across the network. So, simply checking the balance right after submitting a transaction might give you the old value. We need to listen for the transaction to be mined and then update the balance.

Why Real-Time Updates Matter

For a smooth user experience, especially in a trading app like a Uniswap clone, real-time updates are crucial. Imagine a user swaps tokens and sees their balance hasn't changed – that could lead to confusion and frustration. By implementing proper balance updating, you ensure your users see the correct information, building trust and making your dApp feel professional.

Key Concepts: Events and Filters

To achieve this, we'll be leveraging two key concepts in Ethers.js: events and filters. Events are essentially notifications emitted by smart contracts when something significant happens, like a token transfer. Filters allow us to listen for specific events, so we're not bombarded with irrelevant information. Think of it like subscribing to a specific channel on YouTube rather than watching everything that gets uploaded.

Step-by-Step Guide to Updating Token Balances

Okay, let’s get into the code! Here’s a breakdown of how you can update token balances after a transaction using Ethers.js.

1. Setting Up Your Environment

Before we dive in, make sure you have your development environment set up. You'll need Node.js and npm (or yarn) installed. You should also have Ethers.js and any other necessary libraries (like a library for interacting with your smart contract) in your project. If you haven't already, install Ethers.js using:

npm install ethers

2. Connecting to the Blockchain

First things first, you need to connect to the Ethereum network. Ethers.js makes this easy. You can connect to various providers like Infura, Alchemy, or even a local Ganache instance. Here’s how you can connect using Infura:

const { ethers } = require("ethers");

// Replace with your Infura Project ID
const INFURA_ID = "YOUR_INFURA_PROJECT_ID";

// Connect to the network
const provider = new ethers.providers.InfuraProvider("mainnet", INFURA_ID);

Replace "YOUR_INFURA_PROJECT_ID" with your actual Infura project ID. You can also connect to other networks by changing the first argument ("mainnet" in this case) to the network name (e.g., "rinkeby", "kovan", "goerli").

3. Getting the Token Contract Instance

Next, you need to create an instance of your token contract using its address and ABI (Application Binary Interface). The ABI is a JSON representation of your contract's interface, which Ethers.js uses to understand how to interact with the contract.

// Replace with your token contract address and ABI
const TOKEN_ADDRESS = "YOUR_TOKEN_CONTRACT_ADDRESS";
const TOKEN_ABI = [
  // Your token contract ABI here
];

// Create a contract instance
const tokenContract = new ethers.Contract(TOKEN_ADDRESS, TOKEN_ABI, provider);

Make sure to replace "YOUR_TOKEN_CONTRACT_ADDRESS" with the actual address of your token contract and // Your token contract ABI here with the ABI of your token contract. You can usually find the ABI in the compilation output of your smart contract.

4. Listening for the Transfer Event

Now comes the fun part: listening for the Transfer event. ERC-20 tokens (the most common type of token on Ethereum) emit a Transfer event whenever tokens are transferred between accounts. This event is what we'll use to update the balance.

// Address of the user whose balance you want to track
const userAddress = "USER_WALLET_ADDRESS";

// Create a filter for Transfer events to and from the user
const transferFilter = tokenContract.filters.Transfer(null, userAddress);

// Listen for the Transfer event
tokenContract.on(transferFilter, (from, to, value, event) => {
  console.log("Transfer event detected!");
  console.log("From:", from);
  console.log("To:", to);
  console.log("Value:", value.toString()); // Convert BigNumber to string
  console.log("Event:", event);

  // Update the balance here
  updateBalance(userAddress);
});

In this code:

We define userAddress as the address of the user whose balance we want to track.
We create a transferFilter using tokenContract.filters.Transfer(null, userAddress). This filter tells Ethers.js to listen for Transfer events where the to address is the user's address. If you want to track transfers from the user as well, you can modify the filter accordingly.
We use tokenContract.on(transferFilter, ...) to set up a listener for the Transfer event. The callback function is executed whenever a Transfer event matching the filter is emitted.
Inside the callback, we log the event details (from, to, value) and call an updateBalance function (which we'll define next) to update the user's balance.

5. Implementing the `updateBalance` Function

The updateBalance function is responsible for fetching the latest balance of the user and updating it in your application's state. Here’s how you can implement it:

async function updateBalance(address) {
  try {
    // Get the balance of the user
    const balance = await tokenContract.balanceOf(address);

    // Convert the balance to a human-readable format (e.g., using token decimals)
    const formattedBalance = ethers.utils.formatUnits(balance, 18); // Assuming 18 decimals

    console.log("New balance:", formattedBalance);

    // Update your application's state with the new balance
    // (e.g., using React's setState or a similar mechanism)
    // setBalance(formattedBalance);
  } catch (error) {
    console.error("Error fetching balance:", error);
  }
}

In this function:

We use tokenContract.balanceOf(address) to get the user's balance. This returns a BigNumber object, which is a special type used to represent large numbers in JavaScript.
We use ethers.utils.formatUnits(balance, 18) to convert the BigNumber balance to a human-readable format. The 18 represents the number of decimals for the token (most ERC-20 tokens have 18 decimals, but this can vary).
We log the new balance to the console and then comment out a placeholder for updating your application's state. You'll need to replace this with your actual state management logic (e.g., using setState in React).

6. Handling Initial Balance

It's also a good idea to fetch the initial balance when your component mounts or your application starts. You can simply call the updateBalance function with the user's address when your component initializes.

useEffect(() => {
  updateBalance(userAddress);
}, []); // Empty dependency array means this runs once on mount

7. Putting It All Together

Here’s a complete example that combines all the pieces:

const { ethers } = require("ethers");
import { useEffect, useState } from 'react';

// Replace with your Infura Project ID
const INFURA_ID = "YOUR_INFURA_PROJECT_ID";
// Replace with your token contract address and ABI
const TOKEN_ADDRESS = "YOUR_TOKEN_CONTRACT_ADDRESS";
const TOKEN_ABI = [
  // Your token contract ABI here
];
// Address of the user whose balance you want to track
const userAddress = "USER_WALLET_ADDRESS";

function App() {
  const [balance, setBalance] = useState('0');
  useEffect(() => {
      async function init(){
        // Connect to the network
        const provider = new ethers.providers.InfuraProvider("mainnet", INFURA_ID);
        // Create a contract instance
        const tokenContract = new ethers.Contract(TOKEN_ADDRESS, TOKEN_ABI, provider);
        async function updateBalance(address) {
            try {
              // Get the balance of the user
              const balance = await tokenContract.balanceOf(address);
          
              // Convert the balance to a human-readable format (e.g., using token decimals)
              const formattedBalance = ethers.utils.formatUnits(balance, 18); // Assuming 18 decimals
          
              console.log("New balance:", formattedBalance);
              setBalance(formattedBalance)
              // Update your application's state with the new balance
              // (e.g., using React's setState or a similar mechanism)
              // setBalance(formattedBalance);
            } catch (error) {
              console.error("Error fetching balance:", error);
            }
          }
          // Create a filter for Transfer events to and from the user
          const transferFilter = tokenContract.filters.Transfer(null, userAddress);

          // Listen for the Transfer event
          tokenContract.on(transferFilter, (from, to, value, event) => {
            console.log("Transfer event detected!");
            console.log("From:", from);
            console.log("To:", to);
            console.log("Value:", value.toString()); // Convert BigNumber to string
            console.log("Event:", event);
          
            // Update the balance here
            updateBalance(userAddress);
          });
           // Fetch initial balance
           updateBalance(userAddress);
      }
      init()
  }, []);
    return (
      
        <h1>Token Balance: {balance}</h1>
      
    );
  }

  export default App

Remember to replace the placeholder values with your actual Infura project ID, token contract address, ABI, and user address.

Advanced Tips and Considerations

Optimizing Event Listening

Listening for events is powerful, but it can also be resource-intensive. If you're tracking balances for many users, you might want to consider optimizing your event listeners. For example, you could use a more specific filter to only listen for events that are relevant to your application.

Handling Reorgs

Blockchain reorgs (where the chain reorganizes itself, potentially invalidating past transactions) are rare but can happen. To handle reorgs, you can listen for the block event and re-fetch balances periodically or when a reorg is detected.

Using a Backend Service

For more complex applications, you might want to consider using a backend service to handle event listening and balance updates. This can offload the work from the client-side and improve performance and reliability.

Displaying Loading States

While the balance is being updated, it's a good practice to display a loading state to the user. This lets them know that the balance is being fetched and prevents confusion.

Troubleshooting Common Issues

Balance Not Updating

If the balance isn't updating, double-check the following:

Infura Project ID: Make sure your Infura project ID is correct.
Contract Address and ABI: Verify that the token contract address and ABI are correct.
Event Filter: Ensure your event filter is correctly configured to listen for the Transfer events you're interested in.
Error Handling: Check your console for any errors that might be preventing the balance from updating.

Performance Issues

If you're experiencing performance issues, consider optimizing your event listeners or using a backend service to handle balance updates.

Conclusion

Updating token balances after a transaction is a crucial part of building a smooth and user-friendly dApp. By listening for events and updating balances in real-time, you can ensure your users always see the correct information. It might seem a bit complex at first, but with the power of Ethers.js, it becomes manageable. Keep experimenting, keep learning, and you’ll be building awesome dApps in no time! You got this!