Nuxt Magpie module

magpie.js

const fs = require('fs')
const { URL } = require('url')
const { join } = require('path')
const axios = require('axios')
const consola = require('consola')

const defaults = {
  path: '/_images', // dir where downloaded images will be stored
  extensions: ['jpg', 'jpeg', 'gif', 'png', 'webp'],
  baseUrl: '' // cms url
  // TODO: add option to allow keeping the original folder structure
}

export default function Magpie(moduleOptions) {
  const options = { ...defaults, ...moduleOptions }
  const baseDir = join(this.options.generate.dir, options.path)

  this.nuxt.hook('generate:distCopied', () => {
    if (!fs.existsSync(baseDir)) fs.mkdirSync(baseDir)
  })

  this.nuxt.hook('generate:page', async (page) => {
    const urls = []
    const test = new RegExp('(http(s?):)([/|.|\\w|\\s|-])*\.(?:' + options.extensions.join('|') + ')', 'g')
    const matches = page.html.matchAll(test)
    for (const match of matches) {
      const baseUrl = URL(moduleOptions.baseUrl)
      const url = URL(match[0])
      if (baseUrl.hostname === url.hostname && !urls.find(u => u.href === url.href)) {
        urls.push(url)
      }
    }
    if (!urls.length) return
    consola.info(`${page.route}: Magpie is replacing ${urls.length} images with local copies`)
    return await replaceRemoteImages(page.html, urls).then(html => page.html = html)
  })

  async function replaceRemoteImages(html, urls) {
    await Promise.all(urls.map(async (url) => {
      const ext = '.' + url.pathname.split('.').pop()
      const name = slugify(url.pathname.split(ext).join('')) + ext
      const imgPath = join(baseDir, name)
      return saveRemoteImage(url.href, imgPath)
        .then(() => {
          html = html.split(url.href).join(options.path + '/' + name)
        })
        .catch(e => consola.error(e))
    }))
    return html
  }
}

function saveRemoteImage(url, path) {
  return axios({
    url,
    responseType: 'stream'
  }).then(
    response =>
      new Promise((resolve, reject) => {
        response.data
          .pipe(fs.createWriteStream(path))
          .on('finish', () => resolve())
          .on('error', e => reject(e))
      })
  )
}

// https://gist.github.com/codeguy/6684588
function slugify(text) {
  return text
    .toString()
    .toLowerCase()
    .normalize('NFD')
    .trim()
    .replace('/', '')
    .replace(/\s+/g, '-')
    .replace(/[^\w\-]+/g, '-')
    .replace(/\-\-+/g, '-')
}

If you want, I'd be happy to see what solution you came up with!
As a heads up, each time nuxt runs a full static generation, a new dir with the version number is created, so to get the correct path for each route you can do:

  this.nuxt.hook('export:routeCreated', async ({ route, path, errors }) => {
    const routePath = join(this.options.generate.dir, this.options.generate.staticAssets.versionBase, route)
    const payloadPath = join(routePath, 'payload.js')
    // ex. payloadPath = /dist/_nuxt/static/1592921605/about/payload.js

Thanks for the payload path information, it helped me. After a hard time on RegEx and string manipulation... I've managed to complete a working version. However, it needs more testing to be production ready.