vue-cli/packages/@vue/cli/lib/util/getVersions.js

const semver = require('semver')
const { loadOptions, saveOptions } = require('../options')

let sessionCached

module.exports = async function getVersions () {
  if (sessionCached) {
    return sessionCached
  }

  let latest
  const local = require(`../../package.json`).version
  if (process.env.VUE_CLI_TEST || process.env.VUE_CLI_DEBUG) {
    return (sessionCached = {
      current: local,
      latest: local
    })
  }

  const { latestVersion = local, lastChecked = 0 } = loadOptions()
  const cached = latestVersion
  const daysPassed = (Date.now() - lastChecked) / (60 * 60 * 1000 * 24)

  if (daysPassed > 1) {
    // if we haven't check for a new version in a day, wait for the check
    // before proceeding
    latest = await getAndCacheLatestVersion(cached)
  } else {
    // Otherwise, do a check in the background. If the result was updated,
    // it will be used for the next 24 hours.
    getAndCacheLatestVersion(cached)
    latest = cached
  }

  return (sessionCached = {
    current: local,
    latest
  })
}

// fetch the latest version and save it on disk
// so that it is available immediately next time
async function getAndCacheLatestVersion (cached) {
  const getPackageVersion = require('./getPackageVersion')
  const res = await getPackageVersion('vue-cli-version-marker', 'latest')
  if (res.statusCode === 200) {
    const { version } = res.body
    if (semver.valid(version) && version !== cached) {
      saveOptions({ latestVersion: version, lastChecked: Date.now() })
      return version
    }
  }
  return cached
}