How to use the gatsby-core-utils.createContentDigest function in gatsby-core-utils

const icons = pluginOptions.icons || defaultIcons
  const legacy =
    typeof pluginOptions.legacy !== `undefined` ? pluginOptions.legacy : true

  const cacheBusting =
    typeof pluginOptions.cache_busting_mode !== `undefined`
      ? pluginOptions.cache_busting_mode
      : `query`

  // If icons were generated, also add a favicon link.
  if (srcIconExists) {
    const favicon = icons && icons.length ? icons[0].src : null

    if (cacheBusting !== `none`) {
      iconDigest = createContentDigest(fs.readFileSync(pluginOptions.icon))
    }

    const insertFaviconLinkTag =
      typeof pluginOptions.include_favicon !== `undefined`
        ? pluginOptions.include_favicon
        : true

    if (favicon && insertFaviconLinkTag) {
      headComponents.push(
        
      )
    }

// (there will always be one active connection since the dev server
    // keeps a connection open for hot module reloading so we can't wait
    // for networkidle0)
    //
    // unfortunately, this increases the minimum load time per route to
    // ~500ms, which adds up quickly on large sites; there may be room
    // for optimization here
    logger.info(`visit`, route)
    await page.goto(endpoint(route), { waitUntil: `networkidle2` })
    bar.tick()
  }

  await browser.close()
  logger.resetAdapter()

  cache.hash = createContentDigest(routes)
  cache.timestamp = Date.now()
  await save(cache)

  logger.print(`

  ${green(`ok!`)} a mapping between your application's routes and
      font requirements has been generated; make sure to
      add ${bold(`\`gatsby-plugin-preload-fonts\``)} to your app config

        - ${dim(`output`)} ${bold(cacheFile)}

`)
}

${red(`err`)} could not establish a connection with the dev server
    attempted connection to ${endpoint}

    make sure you've run \`gatsby develop\` and set the
    following env variables (if necessary)

      - ${bold(`HOST`)}          ${dim(`(default: localhost)`)}
      - ${bold(`PORT`)}          ${dim(`(default: 8000)`)}
      - ${bold(`LOG_LEVEL`)}     ${dim(`(default: error)`)}
      - ${bold(`GRAPHQL_PATH`)}  ${dim(`(default: /___graphql)`)}

`)
  }

  const routesHash = createContentDigest(routes)
  if (cache.hash === routesHash) {
    const lastRun = formatRelative(new Date(cache.timestamp), new Date())
    const ok = await logger.confirm(`

  ${blue(`note`)} routes have not changed from the last run; if you haven't
       added any new routes or font requirements since then, you
       should be good to go! would you like to crawl them anyways?

         - ${dim(`last run`)} ${bold(lastRun)}
         - ${dim(`route hash`)} ${bold(cache.hash)}

`)
    if (!ok) process.exit(0)
  }

  return routes