metamask-extension/ui/lib/etherscan-prefix-for-network.js

import * as networkEnums from '../../app/scripts/controllers/network/enums'

/**
 * Gets the etherscan.io URL prefix for a given network ID.
 *
 * @param {string} networkId - The network ID to get the prefix for.
 * @returns {string} The etherscan.io URL prefix for the given network ID.
 */
export function getEtherscanNetworkPrefix(networkId) {
  switch (networkId) {
    case networkEnums.ROPSTEN_NETWORK_ID:
      return 'ropsten.'
    case networkEnums.RINKEBY_NETWORK_ID:
      return 'rinkeby.'
    case networkEnums.KOVAN_NETWORK_ID:
      return 'kovan.'
    case networkEnums.GOERLI_NETWORK_ID:
      return 'goerli.'
    default:
      // also covers mainnet
      return ''
  }
}