cleanArtifacts.js

// Removes all files from outputPath that are not in webpack's output
// @this webpack
function cleanArtifacts () {
  const result = new Set()
  // globs to ignore when deleting
  const cleanIgnore = [ '.gitignore' ]

  this.plugin('done', () => {
    if (result.size > 0) {
      // eslint-disable-next-line no-console
      console.error(
        `\rOutdated artifacts deleted:\n\n${
          Array.from(result).map(artifact => `\t${artifact}`).join('\n')
        }\n`
      )
      result.clear()
    }
  })

  const prefix = `${outputPath}/`

  // const recursiveReaddirAsync =
  //   require('bluebird').promisify(require('recursive-readdir'))

  this.plugin('after-emit', (compilation, callback) =>
    Promise.resolve(compilation.getStats())
    .then(stats =>
      Promise.all([
        // the first argument is not a promise, so it's just like Promise.resolve
        new Set(stats.toJson().assets.map(asset => asset.name)),
        // this is, so the Promise.all will await on it
        recursiveReaddirAsync(outputPath, cleanIgnore)
      ])
    )
    // node4 doesn't have destructuring :(
    // .then(([assets, files]) => ...)
    .then(results => {
      const assets = results[0]
      const files = results[1]
      return files.map(file => {
        const artifact = file.replace(prefix, '')

        if (!assets.has(artifact)) {
          return fs.unlinkAsync(file)
          .then(() => { result.add(artifact); return })
        }
        // the .map results in an array of either undefined
        // or Promises that resolve to undefined
      })
    })
    // await on all unlinks
    .then(unlinks => Promise.all(unlinks))
    // the next .then's argument is just array of undefined,
    // but we don't care about the result anyway, just that it resolved :D
    .then(() => callback(), err => callback(err))
  )
}