slotseek-viem

slotseek-viem is a JavaScript library for viem that assists with finding the storage slots for the balanceOf and allowance mappings in an ERC20 token contract, and the permit2 allowance mapping. It also provides a way to generate mock data that can be used to override the state of a contract in an eth_call or eth_estimateGas call.

This is a port of the original slotseek library from ethers.js to viem.

The main use case for this library is to estimate gas costs of transactions that would fail if the address did not have the required balance or approval.

For example, estimating the gas a transaction will consume when swapping, before the user has approved the contract to spend their tokens.

Features

• Find storage slots for balanceOf and allowance mappings in an ERC20 token contract, and permit2 allowance mapping
• Generates mock data that can be used to override the state of a contract in an eth_call/eth_estimateGas call
• Supports vyper storage layouts
• Built for viem with full TypeScript support

How it works

The library uses a brute force approach to find the storage slot of the balanceOf and allowance mappings in an ERC20 token contract. It does this by using a user-provided address that we know has a balance or approval, and then iterates through the storage slots of the contract via the eth_getStorageAt JSON-RPC method until it finds the slot where the storage value matches the user's balance or approval.

This is not a perfect method, and there are more efficient ways to find the storage slot outside of just interacting directly with the contract over RPC. But it's difficult to do so without needing to setup more tools/infra, especially for multi-chain support and gas estimation at runtime. Also, there are not many tools to help with this in javascript.

Installation

npm install @d3or/slotseek-viem
# or
yarn add @d3or/slotseek-viem

TODO

• Add caching options to reduce the number of RPC calls and reduce the time it takes to find the same slot again

Example of overriding a users balance via eth_call

import { createPublicClient, http, parsePrivateKey, privateKeyToAddress, getAddress } from "viem";
import { base } from "viem/chains";
import { generateMockBalanceData } from "@d3or/slotseek-viem";

async function fakeUserBalance() {
  // Setup - Base RPC
  const client = createPublicClient({
    chain: base,
    transport: http("YOUR_RPC_URL")
  });

  // Constants
  const tokenAddress = "0x833589fcd6edb6e08f4c7c32d4f71b54bda02913"; // USDC on Base
  const holderAddress = "0x0000c3Caa36E2d9A8CD5269C976eDe05018f0000"; // USDC holder
  const mockAddress = privateKeyToAddress(parsePrivateKey("0x..." /* generate random */)); // Address to fake balance for
  const mockBalanceAmount = "1000000000000"; // 1 million USDC (6 decimal places), optional. If not provided, defaults to the balance of the holder

  // Generate mock balance data
  const data = await generateMockBalanceData(client, {
    tokenAddress,
    holderAddress,
    mockAddress,
    mockBalanceAmount,
  });

  // Prepare state diff object
  const stateDiff = {
    [tokenAddress]: {
      stateDiff: {
        [data.slot]: data.balance,
      },
    },
  };

  // Make the eth_call with state overrides, or eth_estimateGas
  const balanceOfResponse = await client.request({
    method: 'eth_call',
    params: [
      {
        from: mockAddress,
        to: tokenAddress,
        data: encodeFunctionData({
          abi: [parseAbiItem('function balanceOf(address) view returns (uint256)')],
          functionName: 'balanceOf',
          args: [mockAddress]
        }),
      },
      "latest",
      stateDiff,
    ],
  });

  // Decode and log the result
  const balance = decodeFunctionResult({
    abi: [parseAbiItem('function balanceOf(address) view returns (uint256)')],
    functionName: 'balanceOf',
    data: balanceOfResponse
  });

  console.log(
    `Mocked balance for ${mockAddress}: ${formatUnits(balance, 6)} USDC`
  );
}

fakeUserBalance().catch(console.error);

This can also be used to fake approvals, by using the generateMockApprovalData function instead of generateMockBalanceData.

import { createPublicClient, http, parsePrivateKey, privateKeyToAddress } from "viem";
import { base } from "viem/chains";
import { generateMockApprovalData } from "@d3or/slotseek-viem";

async function fakeUserApproval() {
  // Setup
  const client = createPublicClient({
    chain: base,
    transport: http("YOUR_RPC_URL")
  });

  // Constants
  const tokenAddress = "0x833589fcd6edb6e08f4c7c32d4f71b54bda02913"; // USDC on Base
  const ownerAddress = "0x0000c3Caa36E2d9A8CD5269C976eDe05018f0000"; // USDC holder
  const spenderAddress = "0x000000000022D473030F116dDEE9F6B43aC78BA3"; // Spender address
  const mockAddress = privateKeyToAddress(parsePrivateKey("0x..." /* generate random */)); // Address to fake balance for
  const mockApprovalAmount = "1000000000000"; // 1 million USDC (6 decimal places)

  // Generate mock approval data
  const mockApprovalData = await generateMockApprovalData(client, {
    tokenAddress,
    ownerAddress,
    spenderAddress,
    mockAddress,
    mockApprovalAmount,
  });

  // Prepare state diff object
  const stateDiff = {
    [tokenAddress]: {
      stateDiff: {
        [mockApprovalData.slot]: mockApprovalData.approval,
      },
    },
  };

  // Make the eth_call with state overrides, or eth_estimateGas
  const allowanceResponse = await client.request({
    method: 'eth_call',
    params: [
      {
        from: mockAddress,
        to: tokenAddress,
        data: encodeFunctionData({
          abi: [parseAbiItem('function allowance(address,address) view returns (uint256)')],
          functionName: 'allowance',
          args: [mockAddress, spenderAddress]
        }),
      },
      "latest",
      stateDiff,
    ],
  });

  // Decode and log the result
  const allowance = decodeFunctionResult({
    abi: [parseAbiItem('function allowance(address,address) view returns (uint256)')],
    functionName: 'allowance',
    data: allowanceResponse
  });

  console.log(
    `Mocked allowance for ${mockAddress}: ${formatUnits(allowance, 6)} USDC`
  );
}

fakeUserApproval().catch(console.error);

You can also override both the balance and the allowance at the same time by providing both the balance and approval fields in the state diff object.

Example of just finding the storage slot in a contract

import { createPublicClient, http } from "viem";
import { base } from "viem/chains";
import { getErc20BalanceStorageSlot } from "@d3or/slotseek-viem";

async function findStorageSlot() {
  // Setup - Base RPC
  const client = createPublicClient({
    chain: base,
    transport: http("https://mainnet.base.org")
  });

  // Constants
  const tokenAddress = "0x833589fcd6edb6e08f4c7c32d4f71b54bda02913"; // USDC on Base
  const holderAddress = "0x0000c3Caa36E2d9A8CD5269C976eDe05018f0000"; // USDC holder
  const maxSlots = 100; // Max slots to search

  // Find the storage slot for the balance of the holder
  // or for approvals, use getErc20AllowanceStorageSlot
  const { slot, balance, isVyper } = await getErc20BalanceStorageSlot(
    client,
    tokenAddress,
    holderAddress,
    maxSlots
  );

  console.log(
    `User has balance of ${formatUnits(balance, 6)} USDC stored at slot #${Number(slot)}`
  );
}

findStorageSlot().catch(console.error);

Example of mocking the permit2 allowance mapping

import { createPublicClient, http, pad, toHex } from "viem";
import { base } from "viem/chains";
import { computePermit2AllowanceStorageSlot } from "@d3or/slotseek-viem";

async function findStorageSlot() {
  // Setup - Base RPC
  const client = createPublicClient({
    chain: base,
    transport: http("https://mainnet.base.org")
  });

  // Constants
  const tokenAddress = "0x833589fcd6edb6e08f4c7c32d4f71b54bda02913"; // USDC on Base
  const mockAddress = "0x0000c3Caa36E2d9A8CD5269C976eDe05018f0000"; // USDC holder to mock approval for
  const spenderAddress = "0xd8dA6BF26964aF9D7eEd9e03E53415D37aA96045"

  // Compute storage slot of where the allowance would be held
  const { slot } = computePermit2AllowanceStorageSlot(mockAddress, tokenAddress, spenderAddress)

  const permit2Contract = '0x000000000022d473030f116ddee9f6b43ac78ba3'

  // Prepare state diff object
  const stateDiff = {
    [permit2Contract]: {
      stateDiff: {
        [slot]: pad(
          toHex(BigInt("1461501637330902918203684832716283019655932142975")),
          { size: 32 }
        )
      },
    },
  };

  // Function selector for allowance(address,address,address)
  const allowanceCalldata = encodeFunctionData({
    abi: [parseAbiItem('function allowance(address,address,address) view returns (uint256)')],
    functionName: 'allowance',
    args: [mockAddress, tokenAddress, spenderAddress]
  });

  const allowanceResponse = await client.request({
    method: 'eth_call',
    params: [
      {
        to: permit2Contract,
        data: allowanceCalldata,
      },
      "latest",
      stateDiff
    ],
  });

  // convert the response to a BigNumber
  const approvalAmount = decodeFunctionResult({
    abi: [parseAbiItem('function allowance(address,address,address) view returns (uint256)')],
    functionName: 'allowance',
    data: allowanceResponse
  });

  console.log(
    `Mocked balance for ${mockAddress}: ${formatUnits(approvalAmount, 6)} USDC`
  );
}
findStorageSlot().catch(console.error);