Monitoring swaps on Uniswap with WebSocket endpoints

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

const NODE_URL = "WSS_CHAINSTACK_ENDPOINT";
const web3 = new Web3(NODE_URL);

const UNISWAP_ROUTER_ADDRESS = "0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D"; // Uniswap V2
const SWAP_EXACT_ETH_FOR_TOKENS_SIGNATURE = "0x7ff36ab5";
const SWAP_EXACT_ETH_FOR_FEE_TOKENS_SIGNATURE = "0xb6f9de95";

async function subscribeToNewBlocks() {
  const subscription = await web3.eth.subscribe('newBlockHeaders');
  subscription.on('data', handleNewBlock);
}

async function handleNewBlock(blockHeader) {
  console.log(`Got new block: ${blockHeader.number}`);
  const block = await web3.eth.getBlock(blockHeader.number, true);
  block.transactions.forEach((tx) => {
    if (tx.to && tx.to.toLowerCase() === UNISWAP_ROUTER_ADDRESS.toLowerCase() && (tx.input.startsWith(SWAP_EXACT_ETH_FOR_TOKENS_SIGNATURE) || tx.input.startsWith(SWAP_EXACT_ETH_FOR_FEE_TOKENS_SIGNATURE))) {
      console.log("--------------------------------");
      console.log(`Incoming swap transaction: ${tx.hash}`);
      console.log(`From: ${tx.from}`);
      console.log(`Value: ${web3.utils.fromWei(tx.value, "ether")} ETH`);
      console.log("--------------------------------");
    }
  })
}

subscribeToNewBlocks();

Got new block: 17924098
--------------------------------
Incoming swap transaction: 0x188335bbf730f613cea71a58dad9f9964727f3e93ff934149c83045e8ee0cb36
From: 0xe1605580a0054a99db8d778e560f4b0a2e11a2f2
Value: 0.05 ETH
--------------------------------
--------------------------------
Incoming swap transaction: 0xa9212c6fd39c52048b596c6d2588c63fa5900730c60ffad7f878e5b2b05d2b05
From: 0xd6ea485b803e8473d47985c26220bd1fdc67ec39
Value: 0.1 ETH
--------------------------------
--------------------------------
Incoming swap transaction: 0x442640d4db155f4fef5bfc9b1700d22af596aaec22ca06c392354e150336d3cb
From: 0x551c3dd4271602de4bdee71345bb360ca5f6a9dd
Value: 0.3 ETH
--------------------------------
--------------------------------
Incoming swap transaction: 0x53626667bbfa959af2d8d072f8d1f5a5f0a10074ee177676d794be08d9aaf1db
From: 0xefa316a7faa31356eb6ec9be2c290276f28f6377
Value: 0.014928321838883508 ETH
--------------------------------
--------------------------------
Incoming swap transaction: 0xd6fd0a658051e971a20fe8b241dec11b4b396281b5e0535ed79eef0f18ff1983
From: 0xcde474a2ad4542743007a46863ee2a7906419136
Value: 0.1 ETH
--------------------------------
...
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const { Web3 } = require('web3');

const NODE_URL = "WSS_CHAINSTACK_ENDPOINT";
const web3 = new Web3(NODE_URL);

const UNISWAP_ROUTER_ADDRESS = "0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D"; // Uniswap V2
const SWAP_EXACT_ETH_FOR_TOKENS_SIGNATURE = "0x7ff36ab5";
const SWAP_EXACT_ETH_FOR_FEE_TOKENS_SIGNATURE = "0xb6f9de95";

async function subscribeToNewBlocks() {
  const subscription = await web3.eth.subscribe('newBlockHeaders');
  subscription.on('data', handleNewBlock);
}

async function handleNewBlock(blockHeader) {
  console.log(`Got new block: ${blockHeader.number}`);
  const block = await web3.eth.getBlock(blockHeader.number, true);
  block.transactions.forEach((tx) => {
    if (tx.to && tx.to.toLowerCase() === UNISWAP_ROUTER_ADDRESS.toLowerCase() && (tx.input.startsWith(SWAP_EXACT_ETH_FOR_TOKENS_SIGNATURE) || tx.input.startsWith(SWAP_EXACT_ETH_FOR_FEE_TOKENS_SIGNATURE))) {
      console.log("--------------------------------");
      console.log(`Incoming swap transaction: ${tx.hash}`);
      console.log(`From: ${tx.from}`);
      console.log(`Value: ${web3.utils.fromWei(tx.value, "ether")} ETH`);
      console.log("--------------------------------");
    }
  })
}

subscribeToNewBlocks();

Got new block: 17924098
--------------------------------
Incoming swap transaction: 0x188335bbf730f613cea71a58dad9f9964727f3e93ff934149c83045e8ee0cb36
From: 0xe1605580a0054a99db8d778e560f4b0a2e11a2f2
Value: 0.05 ETH
--------------------------------
--------------------------------
Incoming swap transaction: 0xa9212c6fd39c52048b596c6d2588c63fa5900730c60ffad7f878e5b2b05d2b05
From: 0xd6ea485b803e8473d47985c26220bd1fdc67ec39
Value: 0.1 ETH
--------------------------------
--------------------------------
Incoming swap transaction: 0x442640d4db155f4fef5bfc9b1700d22af596aaec22ca06c392354e150336d3cb
From: 0x551c3dd4271602de4bdee71345bb360ca5f6a9dd
Value: 0.3 ETH
--------------------------------
--------------------------------
Incoming swap transaction: 0x53626667bbfa959af2d8d072f8d1f5a5f0a10074ee177676d794be08d9aaf1db
From: 0xefa316a7faa31356eb6ec9be2c290276f28f6377
Value: 0.014928321838883508 ETH
--------------------------------
--------------------------------
Incoming swap transaction: 0xd6fd0a658051e971a20fe8b241dec11b4b396281b5e0535ed79eef0f18ff1983
From: 0xcde474a2ad4542743007a46863ee2a7906419136
Value: 0.1 ETH
--------------------------------
...
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Overview

Environment setup

Defining web3

To define the web3 variable, you’ll need to first launch a Chainstack Elastic Node, in this case for Ethereum.

Navigate to the Chainstack console

Deploy an Ethereum mainnet node

Open the newly deployed node

Copy the corresponding WSS endpoint

Paste it into your Web3 constructor

JavaScript

const NODE_URL = "WSS_CHAINSTACK_ENDPOINT";
const web3 = new Web3(NODE_URL);

Defining the Uniswap V2 constants

To enable the active filtering of transactions to return ETH -> Token swaps on Uniswap V2, we’ll need to define the following:

UNISWAP_ROUTER_ADDRESS, which as the name suggests, is the address of the V2 router.
SWAP_EXACT_ETH_FOR_TOKENS_SIGNATURE, which is the method ID of the swapExactETHForTokens method.
SWAP_EXACT_ETH_FOR_FEE_TOKENS_SIGNATURE, which is the method ID for the swapExactETHForTokensSupportingFeeOnTransferTokens method.

JavaScript

const UNISWAP_ROUTER_ADDRESS = "0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D"; // Uniswap V2
const SWAP_EXACT_ETH_FOR_TOKENS_SIGNATURE = "0x7ff36ab5";
const SWAP_EXACT_ETH_FOR_FEE_TOKENS_SIGNATURE = "0xb6f9de95";

Opening the subscription

Now, we’ll need to setup the subscription that we’ll use to continually query new blocks as they get confirmed on the Ethereum blockchain.This will be defined as subscribeToNewBlocks, and will open the newBlockHeaders subscription in a variable called subscription.

JavaScript

async function subscribeToNewBlocks() {
  const subscription = await web3.eth.subscribe('newBlockHeaders');
  subscription.on('data', handleNewBlock);
}

Creating the handler function

The final component of this code includes the defining of handleNewBlock, our handler function that will work with the data returned by the subscription.Within this function, we’ll run a getBlock call on the returned block number. This call specifically uses the true flag as the second parameter, which means the getBlock response will include transaction details for each transaction included in the specified block.These transactions will then be looped through and filtered using a conditional that checks for parity with the router address, along with the specified method IDs.

JavaScript

async function handleNewBlock(blockHeader) {
  console.log(`Got new block: ${blockHeader.number}`);
  const block = await web3.eth.getBlock(blockHeader.number, true);
  block.transactions.forEach((tx) => {
    if (tx.to && tx.to.toLowerCase() === UNISWAP_ROUTER_ADDRESS.toLowerCase() && (tx.input.startsWith(SWAP_EXACT_ETH_FOR_TOKENS_SIGNATURE) || tx.input.startsWith(SWAP_EXACT_ETH_FOR_FEE_TOKENS_SIGNATURE))) {
      console.log("--------------------------------");
      console.log(`Incoming swap transaction: ${tx.hash}`);
      console.log(`From: ${tx.from}`);
      console.log(`Value: ${web3.utils.fromWei(tx.value, "ether")} ETH`);
      console.log("--------------------------------");
    }
  })
}

subscribeToNewBlocks();

Understanding the response

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

const NODE_URL = "WSS_CHAINSTACK_ENDPOINT";
const web3 = new Web3(NODE_URL);

const UNISWAP_ROUTER_ADDRESS = "0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D"; // Uniswap V2
const SWAP_EXACT_ETH_FOR_TOKENS_SIGNATURE = "0x7ff36ab5";
const SWAP_EXACT_ETH_FOR_FEE_TOKENS_SIGNATURE = "0xb6f9de95";

async function subscribeToNewBlocks() {
  const subscription = await web3.eth.subscribe('newBlockHeaders');
  subscription.on('data', handleNewBlock);
}

async function handleNewBlock(blockHeader) {
  console.log(`Got new block: ${blockHeader.number}`);
  const block = await web3.eth.getBlock(blockHeader.number, true);
  block.transactions.forEach((tx) => {
    if (tx.to && tx.to.toLowerCase() === UNISWAP_ROUTER_ADDRESS.toLowerCase() && (tx.input.startsWith(SWAP_EXACT_ETH_FOR_TOKENS_SIGNATURE) || tx.input.startsWith(SWAP_EXACT_ETH_FOR_FEE_TOKENS_SIGNATURE))) {
      console.log("--------------------------------");
      console.log(`Incoming swap transaction: ${tx.hash}`);
      console.log(`From: ${tx.from}`);
      console.log(`Value: ${web3.utils.fromWei(tx.value, "ether")} ETH`);
      console.log("--------------------------------");
    }
  })
}

subscribeToNewBlocks();

Got new block: 17924098
--------------------------------
Incoming swap transaction: 0x188335bbf730f613cea71a58dad9f9964727f3e93ff934149c83045e8ee0cb36
From: 0xe1605580a0054a99db8d778e560f4b0a2e11a2f2
Value: 0.05 ETH
--------------------------------
--------------------------------
Incoming swap transaction: 0xa9212c6fd39c52048b596c6d2588c63fa5900730c60ffad7f878e5b2b05d2b05
From: 0xd6ea485b803e8473d47985c26220bd1fdc67ec39
Value: 0.1 ETH
--------------------------------
--------------------------------
Incoming swap transaction: 0x442640d4db155f4fef5bfc9b1700d22af596aaec22ca06c392354e150336d3cb
From: 0x551c3dd4271602de4bdee71345bb360ca5f6a9dd
Value: 0.3 ETH
--------------------------------
--------------------------------
Incoming swap transaction: 0x53626667bbfa959af2d8d072f8d1f5a5f0a10074ee177676d794be08d9aaf1db
From: 0xefa316a7faa31356eb6ec9be2c290276f28f6377
Value: 0.014928321838883508 ETH
--------------------------------
--------------------------------
Incoming swap transaction: 0xd6fd0a658051e971a20fe8b241dec11b4b396281b5e0535ed79eef0f18ff1983
From: 0xcde474a2ad4542743007a46863ee2a7906419136
Value: 0.1 ETH
--------------------------------
...
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