How can make folder with Firebase Cloud Functions Storage

how can I make folder "posts" ?

bucket --> gs://app.appspot.com/posts

code

exports.generateThumbnail = functions.storage.object()
  .onChange(event => {
    const object = event.data
    const filePath = object.name
    const fileName = filePath.split('/').pop()
    const fileBucket = object.bucket;
    const bucket = gcs.bucket(fileBucket)
    const tempFilePath = `/posts/${fileName}`

    return bucket.file(filePath).download({
      destination: tempFilePath
    })
    .then(() => {
      console.log('image downloaded locally to', tempFilePath)
      return spawn('convert', [tempFilePath, '-thumbnail', '400x400>', tempFilePath])
    })
    .then(() => {
      console.log('thumbnail created')
      // match end of string that contains a slash followed by 0 or more characters that are not a slash:
      const thumbFilePath = filePath.replace(/(\/)?([^\/]*)*/, '$1thumb_$2')
          console.log('afoter=thumbFilePath=='+thumbFilePath);
      return bucket.upload(tempFilePath, {
        destination: thumbFilePath
      })
    })

  });

error

Error: EROFS: read-only file system, open '/posts/image.jpg'?