Test you .onCall firebase functions locally.

When you try to test your firebase functions in your local environment, you may face to this

Access to fetch at 'https://[project-id].cloudfunctions.net/[functions-name]' from origin 'http://localhost:5000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: Redirect is not allowed for a preflight request.
POST https://[project-id].cloudfunctions.net/[functions-name] net::ERR_FAILED
Error: internal
    at new y (error.ts:66)
    at w (error.ts:175)
    at A.<anonymous> (service.ts:244)
    at tslib.es6.js:100
    at Object.next (tslib.es6.js:81)
    at r (tslib.es6.js:71)

For you testing on a local environment, you can execute the firebase emulator with these commands

> firebase emulators:start
or
> firebase serve

What I’m introducing way works in both.

/firebase.json

/public/index.html

You have to care about using defer.
If you remove defer keyword from requesting ./index.js it makes an error what cannot find a firebase module.

/functions/index.js

.onCall method is different with .onRequest method.
And you should use a return rather than use “response.send(~)”

/public/index.js

.useFunctionsEmulator function make set you use local functions, not server one.

And I recommend you put that one line for a local testing environment into somewhere like init.js for just the first time once.

And you might be able to use environmental conditions

Using locally emulated https.onCall Firebase functions in locally hosted Firebase application

But this is for server-side. And in your front-end script, you can use this if block

if (location.hostname == "127.0.0.1" || 
  location.hostname == "localhost") {
  console.log('This is local emulator environment')
  functions.useFunctionsEmulator("http://localhost:5001")
}

If you’re using Typescript,

if (location.hostname == "127.0.0.1" || 
  location.hostname == "localhost") {
  console.log('This is local emulator environment')
  firebase.functions().useFunctionsEmulator("http://localhost:5001")
}
}

https://firebase.google.com/docs/reference/android/com/google/firebase/functions/FirebaseFunctions?hl=ja#useEmulator(java.lang.String,%20int)

But, in a google document, they say this method was deprecated. But, the Document is wrong. There is no useEmulator method, not useFunctionsEmulator method.

Just use .useFunctionEmulator method.