subscribe ("pendingTransactions") | Polygon

The ethers.js subscription equivalent to eth_newPendingTransactionFilter. subscribe("pendingTransactions") allows developers to subscribe to real-time updates about pending transactions on the Polygon blockchain; the application will receive notifications whenever a pending transaction appears on the blockchain.

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Parameters

string — a keyword identifying the type of event to subscribe to, pendingTransactions in this case.
function — (optional) a callback function that will be called every time a new event of the specified type is received. This function takes two parameters: error and result. The error parameter contains any error that occurred while subscribing to the event, and the result parameter contains the data for the event that was received.

Response

string — the hash identifying the pending transaction.

Note that ethers.js subscriptions require a WebSocket connection.

Use the provider events to attach listeners to the provider:

"pending" — activates for each new pending transaction.
provider.on — registers an event listener for the specified event.
provider.off — removes a previously registered event listener.
provider.removeAllListeners — removes all listeners for an event and stops the subscription.

const { ethers } = require("ethers");
const NODE_URL = "CHAINSTACK_WSS_URL";
const provider = new ethers.WebSocketProvider(NODE_URL);

async function subscribeToPendingTransactions() {
    try {
        // Subscribe to the 'pending' event to receive new pending transaction hashes
        provider.on("pending", handleNewPending);
        console.log("Subscribed to pending transactions");
    } catch (error) {
        console.error(`Error subscribing to new pending transactions: ${error}`);
    }
}

/* Callback functions to react to the different events */

// Event listener that logs the received pending transaction hashes
function handleNewPending(transactionHash) {
    console.log(transactionHash);
}

// Listen for any provider errors
provider.on("error", handleError);

// Event listener that logs any errors that occur
function handleError(error) {
    console.error(`Error receiving new pending transactions: ${error}`);
}

subscribeToPendingTransactions();

Use case

A practical use case for subscribe("pendingTransactions") is a DApp that continuously listens for new pending transactions, then isolates the from, to, and value fields for analytics purposes. This is useful, for example, to only track transactions that move at least a certain amount of MATIC. The following is an implementation of this concept using ethers.js subscriptions:

const { ethers } = require("ethers");
const NODE_URL = "CHAINSTACK_WSS_URL";
const provider = new ethers.WebSocketProvider(NODE_URL);

async function subscribeToPendingTransactions() {
    try {
        // Subscribe to the 'pending' event to receive new pending transaction hashes
        provider.on("pending", handleNewPending);
        console.log("Subscribed to pending transactions");
    } catch (error) {
        console.error(`Error subscribing to new pending transactions: ${error}`);
    }
}

// Use this function to unsubscribe and exit the program
function unsubscribe() {
    provider.removeAllListeners("pending");
    console.log("Successfully unsubscribed!");
    return process.exit(1);
}

/* Callback functions to react to the different events */

// Event listener that logs the received pending transactions and extracts from, to, and value fields
async function handleNewPending(transactionHash) {
    try {
        const transaction = await provider.getTransaction(transactionHash);
        const from = transaction.from;
        const to = transaction.to;
        const value = transaction.value;
        if (value >= ethers.parseEther("100")) { // 100 MATIC in wei
            console.log(`----- New pending transaction ------`);
            console.log(`From: ${from}`)
            console.log(`To: ${to}`)
            console.log(`Value: ${Number(ethers.formatEther(value)).toFixed(2)} MATIC \n`)
        }
    } catch (error) {
        console.error(`Error getting transaction: ${error}`);
    }
}

// Listen for any provider errors
provider.on("error", handleError);

// Event listener that logs any errors that occur
function handleError(error) {
    console.error(`Error receiving new pending transactions: ${error}`);
}

subscribeToPendingTransactions();

This code creates a new subscription to the pending event by attaching a listener with the provider.on method. Each time a new pending transaction appears, the provider invokes the registered callback with the transaction hash. The code defines two event listener functions: handleNewPending and handleError. The handleNewPending function is called when a new pending transaction is received; it runs the eth_getTransactionByHash method and extracts the from, to, and value fields. If the value transferred is above 100 MATIC, the data is logged. The code includes the unsubscribe function that can be implemented in the logic to unsubscribe and exit the program when a condition is met. It calls provider.removeAllListeners to remove the listeners and stop the subscription. The handleError function is called when an error occurs, and it logs an error message. Finally, the code calls the subscribeToPendingTransactions function, which registers the listener. When a new pending transaction is received, the handleNewPending function is called to extract the data and log it to the console.

​Get your own node endpoint today

​Parameters

​Response

​subscribe("pendingTransactions") code example

​Use case

​Try the subscribe("pendingTransactions") RPC method yourself

Get your own node endpoint today

Parameters

Response

`subscribe("pendingTransactions")` code example

Use case

Try the `subscribe("pendingTransactions")` RPC method yourself