Find allowances slot in an ERC20

whatSlot.js

#!/usr/bin/env node

const { ethers } = require("ethers");
const { formatUnits } = require("@ethersproject/units");
const { program } = require("commander");
const erc20Abi = require("./erc20.json");

const storage = async (
  provider,
  tokenAddress,
  k
) => {
  return ethers.BigNumber.from(
    await provider.getStorageAt(
      tokenAddress,
      k
    )
  );
};

const key = async (
  user,
  contractAddress,
  slot
) => {
  const innerCalc = ethers.utils.solidityKeccak256(
    ["uint256", "uint256"],
    [user, slot]
  );

  return ethers.utils.solidityKeccak256(
    ["uint256", "uint256"],
    [contractAddress, innerCalc]
  );
};

const search = async (
    user,
    tokenAddress,
    target,
    rpc,
    max
  ) => {

    const provider = new ethers.providers.JsonRpcProvider(rpc);
    const token = new ethers.Contract(tokenAddress, erc20Abi, provider);

    let tokenSymbol = tokenAddress;
    let tokenDecimals = 18;
    try {
      tokenDecimals = await token.decimals();
      tokenSymbol = await token.symbol();
    } catch (e) {
      // Fuck it
    }

    // Get the actual allowance
    const allowance = await token.allowance(user, target);

    console.log(`User ${user} approved ${formatUnits(
        allowance,
        tokenDecimals
        )} ${tokenSymbol} tokens to ${target}`);

    for (let i = 0; i <= max; i++) {
      const k = await key(user, target, i);
      const v = await storage(provider, tokenAddress, k);

      if (v.eq(allowance)) {
        console.log(`Slot ${i} of ${tokenSymbol} matches ${allowance}`);
        return;
      }
      else {
        console.log(`Slot ${i} of ${tokenSymbol} did not match ${allowance}`)
      }
    }
};

const main = async () => {
  program
    .argument("<user>", "User address")
    .argument("<token>", "ERC20 token")
    .argument("<target>", "Target Contract")
    .option("--rpc <url>", "RPC URL", "http://127.0.0.1:8545")
    .option("--max <max>", "Max Slot to Test", 15)
    .action(async (user, tokenAddress, target, options) => {

      
      
      await search(
        user,
        tokenAddress,
        target,
        options.rpc,
        parseInt(options.max)
      );
    });
  program.parseAsync();
};

main().catch((e) => console.error(e.toString()));