metamask-extension/app/scripts/lib/createLoggerMiddleware.js

import log from 'loglevel'

/**
 * Returns a middleware that logs RPC activity
 * @param {{ origin: string }} opts - The middleware options
 * @returns {Function}
 */
export default function createLoggerMiddleware (opts) {
  return function loggerMiddleware (/** @type {any} */ req, /** @type {any} */ res, /** @type {Function} */ next) {
    next((/** @type {Function} */ cb) => {
      if (res.error) {
        log.error('Error in RPC response:\n', res)
      }
      if (req.isMetamaskInternal) {
        return
      }
      log.info(`RPC (${opts.origin}):`, req, '->', res)
      cb()
    })
  }
}