metamask-extension/app/scripts/lib/select-chain-id.js

import {
  MAINNET_CHAIN_ID,
  ROPSTEN_CHAIN_ID,
  RINKEBY_CHAIN_ID,
  KOVAN_CHAIN_ID,
  GOERLI_CHAIN_ID,
} from '../controllers/network/enums'

const standardNetworkId = {
  '1': MAINNET_CHAIN_ID,
  '3': ROPSTEN_CHAIN_ID,
  '4': RINKEBY_CHAIN_ID,
  '42': KOVAN_CHAIN_ID,
  '5': GOERLI_CHAIN_ID,
}

export default function selectChainId (metamaskState) {
  const { network, provider: { chainId } } = metamaskState
  return standardNetworkId[network] || `0x${parseInt(chainId, 10).toString(16)}`
}