metamask-extension/ui/helpers/utils/optimism/fetchEstimatedL1Fee.js

import { Contract } from '@ethersproject/contracts';
import { Web3Provider } from '@ethersproject/providers';
import buildUnserializedTransaction from './buildUnserializedTransaction';

// Snippet of the ABI that we need
// Should we need more of it at some point, the full ABI can be found here:
// https://github.com/ethereum-optimism/optimism/blob/develop/gas-oracle/abis/OVM_GasPriceOracle.json
const OPTIMISM_GAS_PRICE_ORACLE_ABI = [
  {
    inputs: [{ internalType: 'bytes', name: '_data', type: 'bytes' }],
    name: 'getL1Fee',
    outputs: [{ internalType: 'uint256', name: '', type: 'uint256' }],
    stateMutability: 'view',
    type: 'function',
  },
];

// BlockExplorer link: https://optimistic.etherscan.io/address/0x420000000000000000000000000000000000000f#code
const OPTIMISM_GAS_PRICE_ORACLE_ADDRESS =
  '0x420000000000000000000000000000000000000F';

export default async function fetchEstimatedL1Fee(txMeta, ethersProvider) {
  const provider = global.ethereumProvider
    ? new Web3Provider(global.ethereumProvider, 10)
    : ethersProvider;
  if (process.env.IN_TEST) {
    provider.detectNetwork = async () => ({
      name: 'optimism',
      chainId: 10,
    });
  }
  const contract = new Contract(
    OPTIMISM_GAS_PRICE_ORACLE_ADDRESS,
    OPTIMISM_GAS_PRICE_ORACLE_ABI,
    provider,
  );
  const serializedTransaction =
    buildUnserializedTransaction(txMeta).serialize();

  const result = await contract.getL1Fee(serializedTransaction);
  return result?.toHexString();
}