> ## Documentation Index
> Fetch the complete documentation index at: https://docs.chainstack.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Monitoring swaps on Uniswap with WebSocket endpoints

> Track Uniswap V2 swap events in near real time using web3.js WebSocket subscriptions and a Chainstack WSS endpoint for live DEX trade monitoring.

<AccordionGroup>
  <Accordion title="Overview">
    This code snippet leverages new block subscriptions, enabled by the usage of a WebSocket endpoint, to watch and sift through new blocks, filter transactions that involve the Uniswap V2 router contract, and then, of which, return those that are ETH -> Token swaps.

    This snippet uses one of the previous recipes on web3.js subscriptions as a base.
  </Accordion>

  <Accordion title="Environment setup">
    If you haven't already, ensure node.js is installed.

    The only dependency here is `web3.js`, which you can install by running:
    `npm install web3`

    ```js JavaScript theme={"system"}
    const { Web3 } = require('web3');
    ```
  </Accordion>

  <Accordion title="Defining web3">
    To define the `web3` variable, you'll need to first launch a Chainstack Elastic Node, in this case for Ethereum.

    <Steps>
      <Step>
        Navigate to the Chainstack console
      </Step>

      <Step>
        Deploy an Ethereum mainnet node
      </Step>

      <Step>
        Open the newly deployed node
      </Step>

      <Step>
        Copy the corresponding WSS endpoint
      </Step>

      <Step>
        Paste it into your `Web3` constructor
      </Step>
    </Steps>

    ```js JavaScript theme={"system"}
    const NODE_URL = "WSS_CHAINSTACK_ENDPOINT";
    const web3 = new Web3(NODE_URL);
    ```
  </Accordion>

  <Accordion title="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:

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

    ```js JavaScript theme={"system"}
    const UNISWAP_ROUTER_ADDRESS = "0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D"; // Uniswap V2
    const SWAP_EXACT_ETH_FOR_TOKENS_SIGNATURE = "0x7ff36ab5";
    const SWAP_EXACT_ETH_FOR_FEE_TOKENS_SIGNATURE = "0xb6f9de95";
    ```
  </Accordion>

  <Accordion title="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`.

    ```js JavaScript theme={"system"}
    async function subscribeToNewBlocks() {
      const subscription = await web3.eth.subscribe('newBlockHeaders');
      subscription.on('data', handleNewBlock);
    }
    ```
  </Accordion>

  <Accordion title="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.

    ```js JavaScript theme={"system"}
    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();
    ```
  </Accordion>

  <Accordion title="Understanding the response">
    For each transaction that meets the criteria within the conditional we set in `handleNewBlock`, a collection of strings will be printed to the console containing key information, such as the transaction ID, the address that executed the swap, and the amount of ETH being swapped.
  </Accordion>
</AccordionGroup>

<RequestExample>
  ```js JavaScript theme={"system"}
  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();
  ```
</RequestExample>

<ResponseExample>
  ```bash theme={"system"}
  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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  See Chainstack in
  ```
</ResponseExample>