TLDR:
  • opBNB is a Layer 2 scalability solution for the BNB Smart Chain built on the Optimism OP Stack.
  • The opBNB Bridge contract emits DepositFinalized events whenever deposits from BNB Chain occur.
  • This tutorial shows how to listen for these deposit events using ethers.js, enabling real-time tracking of cross-chain activity.
  • By monitoring transaction hashes and deposit details, developers can interact directly with the opBNB ecosystem.

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Main article

opBNB is an EVM-compatible Layer 2 scalable network that brings unique features. Developed as an extension of the Binance Smart Chain (BNB Chain) ecosystem, opBNB aims to provide high-performance blockchain solutions. It leverages the bedrock version of the Optimism OP Stack to offer a Layer 2 scaling solution for the BNB Smart Chain. The opBNB network enhances scalability by offloading transaction processing and resource usage from the BNB Smart Chain while posting data to the underlying mainnet. This approach enables high throughput and low fees, making opBNB an attractive choice for developers and users. One of the key components of opBNB is the opBNB Bridge, which facilitates the secure transfer of assets between BNB Chain and opBNB. This bridge is essential for those who need to move assets while maintaining the benefits of both networks. Users interact with the opBNB network by depositing funds from BNB and using applications and contracts on opBNB. This tutorial will guide you through setting up a listener for deposit events on the opBNB Bridge. You can then capture deposit events and extract relevant transaction data. Understanding how to monitor and interact with these on-chain events is crucial for developers leveraging opBNB’s capabilities.

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Prerequisites

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Step-by-step

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Get an opBNB node

Log in to your Chainstack account and deploy a node.

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Create a new ethers project

Once you have Node.js installed, create a new project in a new directory. 
npm init --yes
This will create a new project and a package.json file. You can then install the ethers library. 
npm i ethers
Learn more about Node projects for Web3 by reading Web3 node.js: From zero to a full-fledged project.

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Create the script

Now that the project is set, create a new file named index.js. With this DApp, we listen to the DepositFinalized event on the opBNB Bridge smart contract, 0x4200000000000000000000000000000000000010 in this case. This even is emitted when a deposit is made, you can use this data to monitor activity. Note that this address is a proxy contract. To find the contract event to use in the code, check the actual bridge contract: 0xc0d3c0d3c0d3c0d3c0d3c0d3c0d3c0d3c0d30010. The event is defined in the contract as the following: 
event DepositFinalized(
    address indexed l1Token,
    address indexed l2Token,
    address indexed from,
    address to,
    uint256 amount,
    bytes extraData
);
Paste the following code. 
const { ethers } = require("ethers");

// Connect to your opBNB node
const provider = new ethers.JsonRpcProvider(
  "YOUR_CHAINSTACK_NODE"
);

// Define the contract ABI
const abi = [
  "event DepositFinalized(address indexed l1Token, address indexed l2Token, address indexed from, address to, uint256 amount, bytes extraData)",
];

// Define the contract address
const contractAddress = "0x4200000000000000000000000000000000000010";

// Create a contract instance
const contract = new ethers.Contract(contractAddress, abi, provider);

// Define the event filter (optional: add any specific filters if required)
const filter = contract.filters.DepositFinalized();

console.log("Listening for DepositFinalized events on the opBNB Bridge...");

// Listen for the event
contract.on(filter, (event) => {
  console.log("Deposit Finalized Event:");
  console.log(`Transaction Hash: ${event.log.transactionHash}`);
  console.log(
    `Explorer link: https://opbnbscan.com/tx/${event.log.transactionHash}`
  );
  console.log(`From: ${event.args[2]}`); // from
  console.log(`Amount: ${ethers.formatEther(event.args[4].toString())} BNB`); // amount

  console.log(
    "------------------------------------------------------------------------------------------"
  );
});
where
  • YOUR_CHAINSTACK_NODE — your opBNB node endpoint

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Code breakdown

Here’s a brief breakdown explaining the code step-by-step:

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1. Importing Ethers.js Library

The code starts by importing the Ethers.js library, which is essential for interacting with the Ethereum network and its derivatives like opBNB.

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2. Connecting to the opBNB Node

Using a JSON-RPC provider, it connects to an opBNB node. Replace "YOUR_CHAINSTACK_NODE" with your actual Chainstack node URL. 
const { ethers } = require("ethers");

// Connect to your opBNB node
const provider = new ethers.JsonRpcProvider(
  "YOUR_CHAINSTACK_NODE"
);

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3. Defining the Contract ABI

The ABI (Application Binary Interface) is defined for the DepositFinalized event. This ABI tells Ethers.js how to interpret the event data. 
// Define the contract ABI
const abi = [
  "event DepositFinalized(address indexed l1Token, address indexed l2Token, address indexed from, address to, uint256 amount, bytes extraData)",
];

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4. Specifying the Contract Address

The address of the opBNB Bridge contract is specified. This address points to the smart contract on the opBNB network that emits the DepositFinalized events. 
// Define the contract address
const contractAddress = "0x4200000000000000000000000000000000000010";

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5. Creating a Contract Instance

The contract address, ABI, and provider are used to create an instance of the contract. This instance allows interaction with the contract and listening for events. 
// Create a contract instance
const contract = new ethers.Contract(contractAddress, abi, provider);

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6. Defining the Event Filter

A filter for the DepositFinalized event is defined. If needed, this filter can be customized to listen for specific events based on additional parameters. 
// Define the event filter (optional: add any specific filters if required)
const filter = contract.filters.DepositFinalized();

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7. Handling the Event

The script sets up an event listener that triggers when a DepositFinalized event is detected.
  • Logging Event Details: It logs the event details, such as the transaction hash, which can be used to view the transaction on the opBNB block explorer.
  • Extracting Event Data: The from address and the amount of BNB deposited are extracted from the event arguments and logged. The amount is formatted to be readable in BNB units.
// Listen for the event
contract.on(filter, (event) => {
  console.log("Deposit Finalized Event:");
  console.log(`Transaction Hash: ${event.log.transactionHash}`);
  console.log(
    `Explorer link: https://opbnbscan.com/tx/${event.log.transactionHash}`
  );
  console.log(`From: ${event.args[2]}`); // from
  console.log(`Amount: ${ethers.formatEther(event.args[4].toString())} BNB`); // amount

  console.log(
    "------------------------------------------------------------------------------------------"
  );
});
The script listens for DepositFinalized events on the opBNB Bridge and prints relevant details to the console whenever such an event is emitted by the smart contract.

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Conclusion

By following this tutorial, you have successfully set up a listener for DepositFinalized events on the opBNB Bridge. This allows you to capture and analyze deposit events in real-time, providing valuable insights into the activity on the opBNB network.

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About the author

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Davide Zambiasi

Developer Advocate @ Chainstack BUIDLs on EVM, The Graph protocol, and Starknet Helping people understand Web3 and blockchain development