How can I "admin.initializeApp();" no arguments in local - firebase

Question

I am always grateful for your help.
I want to write code admin.initializeApp(); both locally and in production.
When I deploy functions to production with no auguments, it works.
But locally, it requires me to write it like below:
const serviceAccount = require("/home/yhirochick/development/ServiceAccountKey.json");
admin.initializeApp({
credential: admin.credential.cert(serviceAccount),
databaseURL: "https://xxxx.firebaseio.com/"
});
In the official documentation it says that configuration is applied automatically when you initialize the Firebase Admin SDK with no arguments
But when I execute the command firebase serve --only functions locally and some calls some requests by postman produce the error below:
[2019-07-22T06:45:26.227Z] #firebase/database: FIREBASE WARNING: Provided
authentication credentials for the app named "[DEFAULT]" are invalid. This
usually indicates your app was not initialized correctly. Make sure the
"credential" property provided to initializeApp() is authorized to access the
specified "databaseURL" and is from the correct project.
I want to know How can I "admin.initializeApp();" no arguments locally.

Answer 1

I have grappled with this also and I don't think the local testing scenario currently is explained very well in the official documentation. But here is a solution:
For your local environment you need to download the firebase projects firebase service account json file (found in firebase console under project settings -> service account) and set an environment variable GOOGLE_APPLICATION_CREDENTIALS to point to the file:
# Linux/MACOS version
export GOOGLE_APPLICATION_CREDENTIALS="[PATH_TO_YOUR_SERVICE_ACCOUNT_FILE]"
Read more here, also on how to do this on Windows
Now you will be able to use admin.initializeApp() (with no arguments) locally.
A possible downside of this approach is that you have to set the environment variable each time you fire up a terminal before you start the firebase emulator, because the variable gets deleted when you end the session.
Automate it...
You could automate the export ... command by bundling it together with the command that fires up the emulator. You could do this by adding an entry to the scripts section of your package.json, e.g.:
"local": "export GOOGLE_APPLICATION_CREDENTIALS='[PATH_TO_YOUR_SERVICE_ACCOUNT_FILE]' && firebase emulators:start --only functions"
Then, in this example, you would only need to type npm run local.
Alternative: provide explicit credentials in local environment only
Look at this example: https://stackoverflow.com/a/47517466/1269280.
It basically use a runtime node environment variable to separate between local and production and then use the explicit way of providing credentials in the local environment only.
This is my preferred way of doing things, as I think it is more portable. It allows me to put the service account file inside my codebase and not deal with its absolute file path.
If you do something like this then remember to to exclude the service account file from your repo! (it contains sensitive info).
Background: difference between production and local service account discovery
The reason that admin.initializeApp() (with no arguments) works out-of-the-box in production is that when you deploy to production, i.e. Firebase Functions, the code ends up in a 'Google managed environment'. In Google managed environments like Cloud Functions, Cloud Run, App Engine.. etc, the admin SDK has access to your applications default service account (the one you downloaded above) and will use that when no credentials are specified.
This is part of Google Clouds Application Default Credentials (ADC) strategy which also applies to firebase functions.
Now, your local environment is not a 'google managed environment' so it doesn't have access to the default service account credentials. To google cloud, your local box is just an external server trying to access your private Firebase ressources. So you need to provide your service account credentials in one of the ways described above.
Before I knew this, I thought that because I was already logged in to firebase via my terminal i.e. firebase login and were able to deploy code to firebase, the firebase emulator would also have the necessary credentials for the firebase admin sdk, but this is not the case.