Cloud Functions: callback is not a function

Question

According to https://cloud.google.com/functions/docs/writing/background

You use background functions when you want to have your Cloud Function invoked indirectly in response to an event, such as a message on a Cloud Pub/Sub topic, a change in a Cloud Storage bucket, or a Firebase event.

And the function paramaters are (data, context, callback): https://cloud.google.com/functions/docs/writing/background#function_parameters

However, when I write a simple function like

exports = module.exports = functions.firestore
.document(path)
.onWrite((change, context, callback) => {

   callback()
   return
})

I get an error that says

TypeError: callback is not a function

Is callback not part of Firestore background functions? The documentation says it is

If not, is there anyway to immediately exit a function?

Doug Stevenson · Accepted Answer · 2019-07-30 08:05:42Z

1

The Firebase API is different than the Google Cloud API. What you linked to was the Cloud API, which accepts a callback parameter. The Firebase API which you are actually using does not. The Firebase API for background functions requires you to return a promise that resolves after all the work is complete. There is no callback to call in that case.

answered Jul 30, 2019 at 8:05

Doug Stevenson

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3 Comments

siefix Over a year ago

Thanks. What type of event is it referring to when it says "or a Firebase event"?

Doug Stevenson Over a year ago

An event represents something that happened in a Firebase or Google Cloud product.

siefix Over a year ago

Okay so for everyone looking at this, don't use the Firebase example they give on the Firebase page, callbacks don't work. Use return null instead of callback(): firebase.google.com/docs/functions/…

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