rsnext/packages/create-next-app/helpers/is-online.ts

import { execSync } from 'child_process'
import dns from 'dns'
import url from 'url'

function getProxy(): string | undefined {
  if (process.env.https_proxy) {
    return process.env.https_proxy
  }

  try {
    const httpsProxy = execSync('npm config get https-proxy')
      .toString()
      .trim()
    return httpsProxy !== 'null' ? httpsProxy : undefined
  } catch (e) {
    return
  }
}

export function getOnline(): Promise<boolean> {
  return new Promise(resolve => {
    dns.lookup('registry.yarnpkg.com', registryErr => {
      if (!registryErr) {
        return resolve(true)
      }

      const proxy = getProxy()
      if (!proxy) {
        return resolve(false)
      }

      const { hostname } = url.parse(proxy)
      if (!hostname) {
        return resolve(false)
      }

      dns.lookup(hostname, proxyErr => {
        resolve(proxyErr == null)
      })
    })
  })
}