metamask-extension/app/scripts/controllers/network/createInfuraClient.js

import { createScaffoldMiddleware, mergeMiddleware } from 'json-rpc-engine'
import createBlockReRefMiddleware from 'eth-json-rpc-middleware/block-ref'
import createRetryOnEmptyMiddleware from 'eth-json-rpc-middleware/retryOnEmpty'
import createBlockCacheMiddleware from 'eth-json-rpc-middleware/block-cache'
import createInflightMiddleware from 'eth-json-rpc-middleware/inflight-cache'
import createBlockTrackerInspectorMiddleware from 'eth-json-rpc-middleware/block-tracker-inspector'
import providerFromMiddleware from 'eth-json-rpc-middleware/providerFromMiddleware'
import createInfuraMiddleware from 'eth-json-rpc-infura'
import BlockTracker from 'eth-block-tracker'

import { NETWORK_TYPE_TO_ID_MAP } from '../../../../shared/constants/network'

export default function createInfuraClient({ network, projectId }) {
  const infuraMiddleware = createInfuraMiddleware({
    network,
    projectId,
    maxAttempts: 5,
    source: 'metamask',
  })
  const infuraProvider = providerFromMiddleware(infuraMiddleware)
  const blockTracker = new BlockTracker({ provider: infuraProvider })

  const networkMiddleware = mergeMiddleware([
    createNetworkAndChainIdMiddleware({ network }),
    createBlockCacheMiddleware({ blockTracker }),
    createInflightMiddleware(),
    createBlockReRefMiddleware({ blockTracker, provider: infuraProvider }),
    createRetryOnEmptyMiddleware({ blockTracker, provider: infuraProvider }),
    createBlockTrackerInspectorMiddleware({ blockTracker }),
    infuraMiddleware,
  ])
  return { networkMiddleware, blockTracker }
}

function createNetworkAndChainIdMiddleware({ network }) {
  if (!NETWORK_TYPE_TO_ID_MAP[network]) {
    throw new Error(`createInfuraClient - unknown network "${network}"`)
  }

  const { chainId, networkId } = NETWORK_TYPE_TO_ID_MAP[network]

  return createScaffoldMiddleware({
    eth_chainId: chainId,
    net_version: networkId,
  })
}