index.js

/**
 * Real-time EVM mempool watcher.
 *
 * Subscribes to `newPendingTransactions` over WebSocket. For each pending tx
 * hash, fetches the full transaction body, applies your configured filter,
 * and prints a structured event line.
 *
 * Run:
 *   CHAIN=base node src/index.js
 *
 * Customize filters in src/filters.js or the FILTER constant below.
 */
import { createPublicClient, webSocket, formatEther, formatGwei } from "viem";
import { getChain } from "./chains.js";
import {
  allOf,
  anyOf,
  minValueWei,
  methodSelector,
  isEthTransfer,
  methodSignature,
} from "./filters.js";

// ─── Configure your filter here. Default: ETH transfers >= 1 native unit
//                                          OR token approvals
//                                          OR DEX swaps. ──────────────────
const FILTER = anyOf(
  allOf(isEthTransfer(), minValueWei(10n ** 18n)),  // >= 1 native unit transferred
  methodSelector("0x095ea7b3"),                     // approve()
  methodSelector("0x7ff36ab5"),                     // swapExactETHForTokens
  methodSelector("0x18cbafe5"),                     // swapExactTokensForETH
  methodSelector("0x38ed1739"),                     // swapExactTokensForTokens
  methodSelector("0x3593564c"),                     // Universal Router execute()
);

async function main() {
  const chain = getChain();
  const client = createPublicClient({ transport: webSocket(chain.wsUrl) });

  console.log(`Mempool watcher`);
  console.log(`  Chain:     ${chain.name} (${chain.slug})`);
  console.log(`  WebSocket: ${chain.wsUrl.replace(/key=[^&]+/, "key=***")}`);
  console.log();
  console.log(`Waiting for pending txs... Ctrl-C to stop.`);
  console.log();

  let total = 0;
  let printed = 0;
  let lastReport = Date.now();

  client.watchPendingTransactions({
    onTransactions: async (hashes) => {
      total += hashes.length;
      for (const hash of hashes) {
        try {
          const tx = await client.getTransaction({ hash });
          if (!tx) continue;
          if (!FILTER(tx)) continue;

          const value = formatEther(tx.value ?? 0n);
          const gasPrice = tx.maxFeePerGas
            ? formatGwei(tx.maxFeePerGas) + " gwei (maxFee)"
            : tx.gasPrice
              ? formatGwei(tx.gasPrice) + " gwei"
              : "-";
          const method = methodSignature(tx);
          const methodStr = method
            ? `${method.selector}${method.name ? ` (${method.name})` : ""}`
            : "(eth transfer)";

          printed++;
          console.log(`${new Date().toISOString()}`);
          console.log(`  from:   ${tx.from}`);
          console.log(`  to:     ${tx.to ?? "(contract creation)"}`);
          console.log(`  value:  ${value} ${chain.nativeSymbol}`);
          console.log(`  method: ${methodStr}`);
          console.log(`  gas:    ${gasPrice}`);
          console.log(`  tx:     ${chain.explorerTx}${hash}`);
          console.log();
        } catch {
          /* tx may have already been mined or dropped — skip */
        }
      }

      // Periodic stats line
      const now = Date.now();
      if (now - lastReport > 10_000) {
        const elapsed = (now - lastReport) / 1000;
        const rate = (total / elapsed).toFixed(1);
        process.stderr.write(
          `[stats] ${total} pending seen, ${printed} matched filter (${rate}/s)\n`
        );
        total = 0;
        lastReport = now;
      }
    },
  });
}

main().catch((err) => {
  console.error("Fatal:", err);
  process.exit(1);
});