$ npm init --y
$ npm i express body-parser multer
$ npm i @google-cloud/storage

Follow the instrcutions here: https://cloud.google.com/storage/docs/quickstart-console (in my example I used gcs-demo for the bucket name)

If you want to use a command line:

$ gsutil mb -c regional -l asia-northeast1 gs://gcs-demo/

Since you deploy this app to App Engine (GAE), you don't need to specify a project ID or a service account. GAE will use its default service account called <project_id>@appspot.gserviceaccount.com which already have all the needed persmissions to upload files to GCS. You also don't need to specify the project ID since it runs on GAE.

$ gcloud app deploy

$ gcloud app browse