Test One-Click USDC Checkouts on Starknet with Circle Faucet and SDK

Node.js Script: Wallet Connection, USDC Balance Check, and Transfer Simulation on Starknet Sepolia

This Node.js script connects a Starknet wallet using Starknet.js, queries the USDC balance on Sepolia testnet, and simulates a 1 USDC transfer to estimate fees without executing it. **Caution:** Testnet only. Install `npm i starknet@latest dotenv`. Create `.env` with `PRIVATE_KEY=0x…` and `ACCOUNT_ADDRESS=0x…` (derive from key or deploy Oz account). Fund account with STRK (e.g., Starknet Sepolia faucet) and USDC (Circle Faucet). Verify RPC stability and contract addresses—public RPCs have limits (~100 req/min).

require('dotenv').config();
const { Account, RpcProvider, ec, shortString, uint256 } = require('starknet');

async function main() {
  // ⚠️ Critical: Never hardcode or commit private keys. Use .env for testnet only.
  const privateKey = process.env.PRIVATE_KEY;
  const accountAddress = process.env.ACCOUNT_ADDRESS;
  if (!privateKey || !accountAddress) {
    throw new Error('Set PRIVATE_KEY and ACCOUNT_ADDRESS in .env. Fund via Circle Faucet first.');
  }

  const nodeUrl = 'https://starknet-sepolia.public.blastapi.io/rpc'; // Public RPC; monitor rate limits
  const provider = new RpcProvider({ nodeUrl });

  const keyPair = ec.getKeyPair(privateKey);
  const account = new Account(provider, accountAddress, keyPair);

  // Circle USDC on Starknet Sepolia: Verify from https://developers.circle.com/stablecoins/docs/starknet-sepolia-testnet
  // Placeholder - replace with confirmed address, e.g., 0x04718f5a0fc34cc1af16a1cdee98ffb20c31f5cd61d6ab07201858f4287c938d
  const usdcAddress = '0x04718f5a0fc34cc1af16a1cdee98ffb20c31f5cd61d6ab07201858f4287c938d';

  console.log('🔍 Checking USDC balance for:', account.address);

  // Check balance (read-only call)
  try {
    const { res } = await account.callContract({
      contractAddress: usdcAddress,
      entrypoint: 'balanceOf',
      calldata: [account.address]
    });
    const balance = uint256.uint256ToBN({ low: res[0], high: res[1] });
    const usdcBalance = Number(balance) / 1e6;
    console.log('📊 USDC Balance:', usdcBalance.toFixed(2), 'USDC');
  } catch (error) {
    console.error('❌ Balance check failed:', error.message);
    console.log('💡 Tip: Verify USDC address and account deployment.');
    return;
  }

  // Simulate transfer (fee estimation without execution)
  const recipientAddress = '0x7424b3b5df35a4a9c31e3d3d69e18fe9cca5f5e7d0c2b8c0d5c6e7f8a9b0c1d2'; // Dummy recipient - replace
  const transferAmount = uint256.bnToUint256('1000000'); // 1 USDC (6 decimals)

  const call = {
    contractAddress: usdcAddress,
    entrypoint: 'transfer',
    calldata: [recipientAddress, ...transferAmount]
  };

  try {
    const { suggestedFeeLow, suggestedFeeHigh } = await account.estimateInvokeFee(call);
    const lowFeeEth = Number(suggestedFeeLow) / 1e18;
    console.log('✅ Transfer simulation success');
    console.log('💰 Estimated fee low:', lowFeeEth.toFixed(6), 'ETH (STRK)');
    console.log('💰 Estimated fee high:', Number(suggestedFeeHigh) / 1e18, 'ETH (STRK)');
    console.log('📈 Fees based on current network load; actual may vary 10-20%.');
  } catch (error) {
    console.error('❌ Simulation failed:', error.message);
    if (error.message.includes('balance')) {
      console.log('💡 Low STRK balance for fees or insufficient USDC.');
    } else if (error.message.includes('account')) {
      console.log('💡 Deploy/verify account on Sepolia.');
    }
  }
}

main().catch((error) => {
  console.error('Script error:', error);
  process.exit(1);
});

Run `node check-usdc.js`. Expected: Balance reflects faucet amount (e.g., 100 USDC); fees ~0.00005-0.0005 STRK based on Sepolia data (2024 Q3 averages). Errors? Check logs: 0 balance → refaucet; fee fail → fund STRK. This validates setup before one-click SDK integration. Monitor tx via Voyager Sepolia explorer.