Invoking a Firebase callable function from the Firebase Admin SDK

LOL you can’t.

What you can do however is using the Firebase Admin SDK to create a custom token for the client SDK, and use the client SDK to make the call. 🙃

What does this looks like?

const admin = require('firebase-admin')
const { initializeApp } = require('firebase/app')
const { getAuth, signInWithCustomToken } = require('firebase/auth')
const { getFunctions, httpsCallable } = require('firebase/auth')

admin.initializeApp({
  // Your admin config
})

initializeApp({
  // Your client config
})

const token = await admin.auth().createCustomToken('admin')

await signInWithCustomToken(getAuth(), token)

const result = await httpsCallable(getFunctions(), 'myCallableFunction').call({})

Here we created a custom token for a virtual user with UID admin (it doesn’t need to exist in Firebase Auth). We can verify that in the function:

const functions = require('firebase-functions')

exports.myCallableFunction = functions.https.onCall(async (data, context) => {
  if (!context.auth) {
    throw new functions.https.HttpsError(
      'unauthenticated',
      'User is not authenticated'
    )
  }

  if (context.auth.uid !== 'admin') {
    throw new functions.https.HttpsError(
      'permission-denied',
      'User is not authorized'
    )
  }
})

Why callable instead of HTTP function?

With a HTTP function, we could have made a simple fetch request to the endpoint.

Why use a callable function then?

In my case, it was because callable functions have auth built-in, whereas you’re responsible to implement your own auth for HTTP functions. I found that using a https.onCall function with a custom Firebase token was more elegant than configuring some kind of internal “API key”.

Invoking the callable function manually

It turns out it’s also quite easy to invoke a callable function without the Firebase SDK, via a plain HTTP call, as detailed in this reference.

With cURL, calling a function that doesn’t have token authentication is as simple as:

curl \
    -X POST \
    -H 'Content-Type: application/json' \
    'https://region-project.cloudfunctions.net/myCallableFunction' \
    --data '{"data": {}}'

For the authentication, we need an Authorization: Bearer header, but we can’t directly use the custom token we generated above. We need to exchange it for an ID token first (this happened transparently in the previous example).

We could use the client SDK to do that for us but at that point we might as well use the client SDK to call the function as well. 😅

Just for educational purpose, and building off the earlier example, it would look like:

const { getAuth, signInWithCustomToken, getIdToken } = require('firebase/auth')

await signInWithCustomToken(getAuth(), token)

console.log(await getIdToken(getAuth().currentUser))

We could then use that token in the cURL request:

curl \
    -X POST \
    -H 'Content-Type: application/json' \
    -H "Authorization: Bearer $token" \
    'https://region-project.cloudfunctions.net/myCallableFunction' \
    --data '{"data": {}}'

But if we’re calling the function via fetch, it’s probably that we don’t want to use the client SDK. Then, exchanging the token would look like this:

curl \
    -X POST \
    -H 'Content-Type: application/json' \
    "https://identitytoolkit.googleapis.com/v1/accounts:signInWithCustomToken?key=$firebaseApiKey" \
    --data "{\"token\": \"$customToken\", \"returnSecureToken\": true}"

This returns an idToken that we can use as Authorization: Bearer in the invocation of the callable function as seen above.

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