Cloud Storage triggers

You can trigger a function in response to the uploading, updating, or deleting of files and folders in Cloud Storage.

Examples in this page are based on a sample function that triggers when image files are uploaded to Cloud Storage. This sample function demonstrates how to access event attributes, how to download a file to a Cloud Functions instance, and other fundamentals of handling Cloud Storage events.

For more examples of use cases, see What can I do with Cloud Functions?

Trigger a function on Cloud Storage changes

Use to create a function that handles Cloud Storage events. Depending on whether you want to scope your function to a specific Cloud Storage bucket or use the default bucket, use one of the following:

For example, the thumbnail generator sample is scoped to the default bucket for the project:

exports.firstGenGenerateThumbnail = (object) => {
  // ...

Cloud Storage supports these events:

  • onArchive Only sent when a bucket has enabled object versioning. This event indicates that the live version of an object has become an archived version, either because it was archived or because it was overwritten by the upload of an object of the same name.
  • onDelete Sent when an object has been permanently deleted. This includes objects that are overwritten or are deleted as part of the bucket's lifecycle configuration. For buckets with object versioning enabled, this is not sent when an object is archived (see onArchive), even if archival occurs via the storage.objects.delete method.
  • onFinalize Sent when a new object (or a new generation of an existing object) is successfully created in the bucket. This includes copying or rewriting an existing object. A failed upload does not trigger this event.
  • onMetadataUpdate Sent when the metadata of an existing object changes.

Set the event within the on event handler as shown above for onFinalize.

Access Cloud Storage object attributes

Cloud Functions exposes a number of Cloud Storage object attributes such as size and contentType for the file updated. The 'metageneration' attribute is incremented whenever there's a change to the object's metadata. For new objects, the metageneration value is 1.

const fileBucket = object.bucket; // The Storage bucket that contains the file.
const filePath =; // File path in the bucket.
const contentType = object.contentType; // File content type.

The thumbnail generation sample uses some of these attributes to detect exit cases in which the function returns:

// Exit if this is triggered on a file that is not an image.
if (!contentType.startsWith('image/')) {
  return functions.logger.log('This is not an image.');

// Get the file name.
const fileName = path.basename(filePath);
// Exit if the image is already a thumbnail.
if (fileName.startsWith('thumb_')) {
  return functions.logger.log('Already a Thumbnail.');

Download, transform, and upload a file

For some cases, it may not be necessary to download files from Cloud Storage. However, to perform intensive tasks such as generating a thumbnail image from a file stored in Cloud Storage, you need to download files to the functions instance—that is, the virtual machine that runs your code.

To easily download and re-upload objects to Cloud Storage, install the Google Cloud Storage package using npm install --save @google-cloud/storage, and import it. To use JavaScript promises to handle external processes like the thumbnail processing tasks in the sample, also import child-process-promise:

const functions = require('firebase-functions/v1');
const admin = require('firebase-admin');
const path = require('path');

//library for resizing images
const sharp = require('sharp');

Use gcs.bucket.file(filePath).download to download a file to a temporary directory on your Cloud Functions instance. In this location, you can process the file as needed and then upload to Cloud Storage. When performing asynchronous tasks, make sure you return a JavaScript promise in your callback.

Example: image transformation

Using Cloud Functions together with image-processing programs like sharp, you can perform manipulations on graphical image files. The following is an example of how to create a thumbnail image for an uploaded image file:

// Download file from bucket.
const bucket =;
const metadata = {
  contentType: contentType,
const downloadResponse = await bucket.file(filePath).download();
const imageBuffer = downloadResponse[0];
functions.logger.log("Image downloaded!");

// Generate a thumbnail using sharp.
const thumbnailBuffer = await sharp(imageBuffer).resize({
  width: 200,
  height: 200,
  withoutEnlargement: true,
functions.logger.log("Thumbnail created");

// Upload the thumbnail with a 'thumb_' prefix.
const thumbFileName = `thumb_${fileName}`;
const thumbFilePath = path.join(path.dirname(filePath), thumbFileName);
await bucket.file(thumbFilePath).save(thumbnailBuffer, {
  metadata: metadata,
return functions.logger.log("Thumbnail uploaded!");

This code creates a 200x200 thumbnail for the image saved in a temporary directory, then uploads it back to Cloud Storage.

Explore more examples

More examples of common media transformation functions including transcoding images, moderating content, extracting EXIF metadata. The full list of examples is available on GitHub.