const tx: Transaction.EIP1559 = {
  type: Transaction.Type.EIP1559,
  chainId: 1n,  // Explicit field
  // ...
}

getChainId(tx) // 1n

// EIP-155 formula: v = chainId * 2 + 35 + yParity
// Reverse: chainId = (v - 35 - yParity) / 2

// Example: Mainnet (chainId = 1)
v = 37 → chainId = (37 - 35) / 2 = 1  // yParity = 0
v = 38 → chainId = (38 - 35 - 1) / 2 = 1  // yParity = 1

// Example: Polygon (chainId = 137)
v = 309 → chainId = (309 - 35) / 2 = 137  // yParity = 0
v = 310 → chainId = (310 - 35 - 1) / 2 = 137  // yParity = 1

const preEip155: Transaction.Legacy = {
  type: Transaction.Type.Legacy,
  v: 27n,  // yParity = 0, no chain ID
  // ...
}

getChainId(preEip155) // undefined

import { getChainId } from 'tevm/Transaction'
import { TransactionError } from 'tevm/errors'

function validateChainId(tx: Transaction.Any, expectedChainId: bigint) {
  const txChainId = getChainId(tx)

  if (txChainId !== undefined && txChainId !== expectedChainId) {
    throw new TransactionError(
      `Wrong chain: expected ${expectedChainId}, got ${txChainId}`,
      {
        code: 'WRONG_CHAIN_ID',
        context: { expected: expectedChainId, actual: txChainId }
      }
    )
  }
}

// Usage
validateChainId(tx, 1n) // Mainnet

import { getChainId } from 'tevm/Transaction'

function detectChain(tx: Transaction.Any): string {
  const chainId = getChainId(tx)

  switch (chainId) {
    case 1n: return 'Ethereum Mainnet'
    case 137n: return 'Polygon'
    case 42161n: return 'Arbitrum One'
    case 10n: return 'Optimism'
    default: return chainId ? `Chain ${chainId}` : 'Pre-EIP-155'
  }
}

import { getChainId } from 'tevm/Transaction'

function filterByChain(
  transactions: Transaction.Any[],
  targetChainId: bigint
): Transaction.Any[] {
  return transactions.filter(tx => {
    const chainId = getChainId(tx)
    return chainId === targetChainId
  })
}

// Get all mainnet transactions
const mainnetTxs = filterByChain(allTransactions, 1n)