Firebase Callable Function + CORS

For anyone looking at this post Jan 2020..

Found this answer on Reddit (https://www.reddit.com/r/reactjs/comments/fsw405/firebase_cloud_functions_cors_policy_error/)

Turns out on 15th Jan 2020, Google changed the security settings where all new functions no longer have a Cloud Functions Invoker. This means that all newly created functions will have their access forbidden, thus resulting in a CORS policy block.

Here is how you fix it, as it's not all that obvious:

https://cloud.google.com/functions/docs/securing/managing-access-iam#allowing_unauthenticated_function_invocation

For anybody else who has arrived here searching firebase callable functions cors errors, here's my checklist:

1. Ensure the function is deployed.
2. Ensure the function name is correct. I was calling recalculatY when it should have been recalculateY. Got a cors error for some reason.
3. Ensure the function code itself is not throwing an error. Use the emulator to help. This didn't throw a cors error still helpful to know.
4. Ensure your regions match - I am using europe-west2. I had to both deploy the function with that region, and call it using the region. For a while, I assumed the client would infer the correct region if the function name was correct. That was not the case.
5. This last point is easy to oversee as everything else in the Firebase universe is more or less plug-and-play, but you need to make sure your functions are accessible by giving them the right permission. (This point is taken from @kitson's answer below. As many only read the first answer I think it is important to highlight this.)

Deploying a callable function to a specific region:

// This is the file in which you define your callable function.
const functions = require('firebase-functions');
...
exports.yourFunc = functions.region('europe-west2').https.onCall(async (data, context) => {
    ...
})

Calling a function in a specific region from the client (in this case, a vuejs web app):

// In my case, this is a vuex store file, but it is safe to assume this is plain old javascript
import firebase from 'firebase/app'
import 'firebase/functions'
...
firebase.app().functions('europe-west2').httpsCallable('yourFunc')

Note: firebase.app().function... vs firebase.app.function...