How to pay gas with any token

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const { ethers } = require("ethers");
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// --- Constants ---
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// Address of the ERC-4337 EntryPoint contract
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const ENTRYPOINT_ADDRESS = "0x5FF137D4b0FDCD49DcA30c7CF57E578a026d2789";
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// ABI for the EntryPoint contract, specifically for the getNonce function
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const ENTRYPOINT_ABI = [
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  {
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    type: "function",
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    name: "getNonce",
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    inputs: [
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      { name: "sender", type: "address", internalType: "address" },
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      { name: "key", type: "uint192", internalType: "uint192" },
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    ],
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    outputs: [
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      {
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        name: "nonce",
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        type: "uint256",
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        internalType: "uint256",
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      },
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    ],
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    stateMutability: "view",
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  },
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];
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// Alchemy RPC URL for Sepolia testnet
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const ALCHEMY_RPC_URL = "";
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// Alchemy Gas Manager RPC URL for Sepolia testnet
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const ALCHEMY_GAS_MANAGER_URL = "";
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// Policy ID for the Alchemy Gas Manager
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const ALCHEMY_POLICY_ID = "";
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// Address of the ERC20 token to be used for gas payment
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const ERC20_TOKEN_ADDRESS = "0x1c7d4b196cb0c7b01d743fbc6116a902379c7238"; // USDC
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// --- Ethers.js Setup ---
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// Initialize a JSON RPC provider
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const provider = new ethers.JsonRpcProvider(ALCHEMY_RPC_URL);
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// Create an ethers.js contract instance for the EntryPoint contract
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const entryPoint = new ethers.Contract(
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  ENTRYPOINT_ADDRESS,
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  ENTRYPOINT_ABI,
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  provider,
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);
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// --- Alchemy API Functions ---
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/**
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 * Requests gas fee estimations and paymaster data from Alchemy.
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 * This function constructs and sends a request to the 'alchemy_requestGasAndPaymasterAndData' RPC method.
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 * @param {object} uo - The user operation object.
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 * @returns {Promise<object>} A promise that resolves to the result object from Alchemy,
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 *                            containing paymasterAndData and gas estimations.
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 */
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async function requestGasAndPaymaster(uo) {
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  const body = JSON.stringify({
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    id: 1,
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    jsonrpc: "2.0",
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    method: "alchemy_requestGasAndPaymasterAndData",
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    params: [
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      {
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        policyId: ALCHEMY_POLICY_ID,
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        userOperation: {
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          sender: uo.sender,
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          nonce: uo.nonce,
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          initCode: uo.initCode,
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          callData: uo.callData,
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        },
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        erc20Context: {
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          tokenAddress: ERC20_TOKEN_ADDRESS,
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        },
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        entryPoint: ENTRYPOINT_ADDRESS,
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        dummySignature: uo.signature,
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      },
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    ],
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  });
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  const options = {
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    method: "POST",
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    headers: { accept: "application/json", "content-type": "application/json" },
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    body,
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  };
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  const res = await fetch(ALCHEMY_GAS_MANAGER_URL, options);
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  const jsonRes = await res.json();
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  console.log("Alchemy Gas and Paymaster Response:", jsonRes);
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  return jsonRes.result;
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}
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/**
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 * Sends a user operation to the bundler via Alchemy.
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 * This function constructs and sends a request to the 'eth_sendUserOperation' RPC method.
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 * @param {object} uo - The complete user operation object, including paymaster data and gas estimations.
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 * @returns {Promise<void>} A promise that resolves when the request is sent.
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 *                          The bundler's response (including userOpHash) is logged to the console.
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 */
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async function sendUserOperation(uo) {
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  const body = JSON.stringify({
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    id: 1,
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    jsonrpc: "2.0",
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    method: "eth_sendUserOperation",
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    params: [uo, ENTRYPOINT_ADDRESS],
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  });
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  const options = {
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    method: "POST",
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    headers: { accept: "application/json", "content-type": "application/json" },
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    body,
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  };
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  const res = await fetch(ALCHEMY_GAS_MANAGER_URL, options);
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  const jsonRes = await res.json();
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  console.log("Alchemy Send UserOperation Response:", jsonRes);
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}
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// --- Main Script Execution ---
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// Define the initial user operation object
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// This object contains the core details of the transaction to be executed.
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const userOp = {
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  sender: "0x", // Smart account address
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  nonce: "0x", // Initial nonce (will be updated)
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  initCode: "0x", // Set to "0x" if the smart account is already deployed
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  callData: "0x", // Encoded function call data
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  signature: "0x", // Dummy signature, should be replaced after requesting paymaster data
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};
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// IIFE (Immediately Invoked Function Expression) to run the async operations
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(async () => {
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  // Fetch the current nonce for the sender address from the EntryPoint contract
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  const nonce = BigInt(await entryPoint.getNonce(userOp.sender, 0));
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  userOp.nonce = "0x" + nonce.toString(16); // Update userOp with the correct nonce
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  console.log("Fetching paymaster data and gas estimates...");
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  // Request paymaster data and gas estimations from Alchemy
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  const paymasterAndGasData = await requestGasAndPaymaster(userOp);
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  // Combine the original userOp with the data returned by Alchemy (paymasterAndData, gas limits, etc.)
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  const userOpWithGas = { ...userOp, ...paymasterAndGasData }; // Spread order matters: userOp properties can be overwritten by paymasterAndGasData if names clash (e.g. if userOp had gas fields)
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  console.log(
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    "Final UserOperation with Gas and Paymaster Data:",
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    JSON.stringify(userOpWithGas, null, 2),
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  );
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  console.log("EntryPoint Address used for submission: ", ENTRYPOINT_ADDRESS);
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  // The script currently stops here. Uncomment the line below to actually send the UserOperation.
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  // Make sure your account is funded with the ERC20 token and has approved the paymaster.
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  return; // Intentionally stopping before sending for review. Remove this line to proceed.
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  // userOpWithGas.signature = await sign(userOpWithGas);
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  // await sendUserOperation(userOpWithGas);
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})();