Function getFallbackEnvs

build-fallback-worker.js:9–55

({ fallbacks, basedir, id, pageExtensions })

const TerserPlugin = require('terser-webpack-plugin')

const getFallbackEnvs = ({ fallbacks, basedir, id, pageExtensions }) => {
  let { document, data } = fallbacks

  if (!document) {
    let pagesDir = undefined

    if (fs.existsSync(path.join(basedir, 'pages'))) {
      pagesDir = path.join(basedir, 'pages')
    } else if (fs.existsSync(path.join(basedir, 'src', 'pages'))) {
      pagesDir = path.join(basedir, 'src', 'pages')
    }

    if (!pagesDir) return

    const offlines = pageExtensions
      .map(ext => path.join(pagesDir, `_offline.${ext}`))
      .filter(entry => fs.existsSync(entry))
    if (offlines.length === 1) {
      document = '/_offline'
    }
  }

  if (data && data.endsWith('.json')) {
    data = path.posix.join('/_next/data', id, data)
  }

  const envs = {
    __PWA_FALLBACK_DOCUMENT__: document || false,
    __PWA_FALLBACK_IMAGE__: fallbacks.image || false,
    __PWA_FALLBACK_AUDIO__: fallbacks.audio || false,
    __PWA_FALLBACK_VIDEO__: fallbacks.video || false,
    __PWA_FALLBACK_FONT__: fallbacks.font || false,
    __PWA_FALLBACK_DATA__: data || false
  }

  if (Object.values(envs).filter(v => !!v).length === 0) return

  console.log('> [PWA] Fallback to precache routes when fetch failed from cache or network:')
  if (envs.__PWA_FALLBACK_DOCUMENT__) console.log(`> [PWA]   document (page): ${envs.__PWA_FALLBACK_DOCUMENT__}`)
  if (envs.__PWA_FALLBACK_IMAGE__) console.log(`> [PWA]   image: ${envs.__PWA_FALLBACK_IMAGE__}`)
  if (envs.__PWA_FALLBACK_AUDIO__) console.log(`> [PWA]   audio: ${envs.__PWA_FALLBACK_AUDIO__}`)
  if (envs.__PWA_FALLBACK_VIDEO__) console.log(`> [PWA]   video: ${envs.__PWA_FALLBACK_VIDEO__}`)
  if (envs.__PWA_FALLBACK_FONT__) console.log(`> [PWA]   font: ${envs.__PWA_FALLBACK_FONT__}`)
  if (envs.__PWA_FALLBACK_DATA__) console.log(`> [PWA]   data (/_next/data/**/*.json): ${envs.__PWA_FALLBACK_DATA__}`)

  return envs
}

const buildFallbackWorker = ({ id, fallbacks, basedir, destdir, minify, pageExtensions }) => {
  const envs = getFallbackEnvs({ fallbacks, basedir, id, pageExtensions })