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

import createAsyncMiddleware from 'json-rpc-engine/src/createAsyncMiddleware'
import mergeMiddleware from 'json-rpc-engine/src/mergeMiddleware'
import createFetchMiddleware from 'eth-json-rpc-middleware/fetch'
import createBlockRefRewriteMiddleware from 'eth-json-rpc-middleware/block-ref-rewrite'
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 BlockTracker from 'eth-block-tracker'

const inTest = process.env.IN_TEST === 'true'
const blockTrackerOpts = inTest ? { pollingInterval: 1000 } : {}
const getTestMiddlewares = () => {
  return inTest ? [createEstimateGasDelayTestMiddleware()] : []
}

export default function createJsonRpcClient({ rpcUrl, chainId }) {
  const fetchMiddleware = createFetchMiddleware({ rpcUrl })
  const blockProvider = providerFromMiddleware(fetchMiddleware)
  const blockTracker = new BlockTracker({
    ...blockTrackerOpts,
    provider: blockProvider,
  })

  const networkMiddleware = mergeMiddleware([
    ...getTestMiddlewares(),
    createChainIdMiddleware(chainId),
    createBlockRefRewriteMiddleware({ blockTracker }),
    createBlockCacheMiddleware({ blockTracker }),
    createInflightMiddleware(),
    createBlockTrackerInspectorMiddleware({ blockTracker }),
    fetchMiddleware,
  ])

  return { networkMiddleware, blockTracker }
}

function createChainIdMiddleware(chainId) {
  return (req, res, next, end) => {
    if (req.method === 'eth_chainId') {
      res.result = chainId
      return end()
    }
    return next()
  }
}

/**
 * For use in tests only.
 * Adds a delay to `eth_estimateGas` calls.
 */
function createEstimateGasDelayTestMiddleware() {
  return createAsyncMiddleware(async (req, _, next) => {
    if (req.method === 'eth_estimateGas') {
      await new Promise((resolve) => setTimeout(resolve, 2000))
    }
    return next()
  })
}