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WTF Ethers: 19. Listen to Mempool

I've been revisiting ethers.js recently to refresh my understanding of the details and to write a simple tutorial called "WTF Ethers" for beginners.

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All the code and tutorials are open-sourced on GitHub:

In this lesson, we will learn how to read transactions from the mempool.


MEV (Maximal Extractable Value) is a fascinating topic. Most people are unfamiliar with it as it did not exist before the invention of blockchain that supports smart contracts. It's a feast for searchers, a friend of miners, and a nightmare for retail investors.

In blockchain, miners/validators can profit by packing, excluding, or reordering transactions in the blocks they generate, and MEV is a metric that measures this profit.


Before a user's transaction is included in the Ethereum blockchain by miners/validators, all transactions gather in the Mempool. Miners also search for transactions with high fees in the Mempool to prioritize packaging and maximize their profits. Generally, transactions with higher gas prices are more likely to be packaged.

At the same time, some MEV bots also search for profitable trades in the mempool. For example, a swap transaction with a high slippage may be subject to sandwich attacks: the bot adjusts the gas price to insert a buy order before the transaction, and then sends a sell order afterward, effectively selling the tokens at a higher price to the user (front-running).


Listening to the Mempool

You can use the Provider class provided by ethers.js to listen to pending transactions in the mempool:

provider.on("pending", listener)

Mempool Listening Script

Below is a script that listens to the mempool.

  1. Create the provider and wallet. This time, we will use a WebSocket Provider for more persistent transaction listening. Thus, we need to change the url to wss.

    console.log("\n1. Connect to wss RPC")
    // Prepare alchemy API, for reference visit:
    const ALCHEMY_MAINNET_WSSURL = 'wss://';
    const provider = new ethers.WebSocketProvider(ALCHEMY_MAINNET_WSSURL);
  2. Because there are many pending transactions in the mempool, sometimes hundreds per second, it is easy to reach the request limit of free RPC nodes. Therefore, we need to throttle the request frequency using throttle.

    function throttle(fn, delay) {
    let timer;
    return function(){
    if(!timer) {
    fn.apply(this, arguments)
    timer = setTimeout(()=>{
    timer = null
  3. Listen to the pending transactions in the mempool and print the transaction hash.

    let i = 0
    provider.on("pending", async (txHash) => {
    if (txHash && i < 100) {
    // Print txHash
    console.log(`[${(new Date).toLocaleTimeString()}] Listening to Pending Transaction ${i}: ${txHash} \r`);

    Get pending transaction hash

  4. Get the transaction details using the hash of the pending transaction. We can see that the transaction has not been included in a block yet, so its blockHash, blockNumber, and transactionIndex are all empty. However, we can retrieve information such as the sender address from, gas price gasPrice, target address to, amount of ether sent value, transaction data data, etc. Bots utilize this information for MEV mining.

    let j = 0
    provider.on("pending", throttle(async (txHash) => {
    if (txHash && j >= 100) {
    // Get transaction details
    let tx = await provider.getTransaction(txHash);
    console.log(`\n[${(new Date).toLocaleTimeString()}] Listening to Pending Transaction ${j}: ${txHash} \r`);
    }, 1000));

    Get transaction details


In this lesson, we briefly introduced MEV and mempool, and wrote a script to listen to pending transactions in the mempool.