Setting NODE_ENV for firebase function

There is currently no way to set custom environment variables such as process.env.NODE_ENV. What you want to do can only be done for Google Cloud functions and you need to use the gcloud command-line tool.

https://cloud.google.com/functions/docs/env-var#accessing_environment_variables_at_runtime

Other options

If you are developing specifically for Firebase and need a similar solution, then there are options.

Conditions based on project ID

You can access the project id if you are having test, staging and production projects and want to have different behavior or logging depending on the environment.

process.env.GCLOUD_PROJECT is set to your GCP project ID so you can build logic based on that.

if (process.env.GCLOUD_PROJECT === 'my-production-project') {
  // Only in production
} else {
  // Do something for the test environments
}

Cloud Function Environment Variables

As you already mentioned there's the cloud functions environment variables also. You can effectively create build pipelines that are configuring your environment configuration upon build/deploy and later access them in your cloud function.

- firebase functions:config:set runtime.env="production" --token $FIREBASE_DEPLOY_KEY

Accessing the configuration is effectively the same as your process.env but can not be accessed outside of the scope of a cloud function (i.e. you can't use it in a global variable declaration).

if (functions.config().runtime.env === 'production') {
  // Only in production
} else {
  // Do something for the test environments
}

Following Google's Best practices and reserved environment variables in their documentation

Environment variables that are provided by the environment might change in future runtime versions. As a best practice, we recommend that you do not depend on or modify any environment variables that you have not set explicitly.

Basically don't use NODE_ENV. Use your own environment variables and set them accordingly.

NOTE: This documentations is from Google Cloud Functions. Firebase functions are like a wrapper around google cloud functions. Check this question

At the time I'm answering this question the Firebase SDK for Cloud Functions offers built-in environment configuration out of the box.

Set environment configuration for your project

$ firebase functions:config:set [values...]

Example

$ firebase functions:config:set someservice.key="THE API KEY" someservice.id="THE CLIENT ID"

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Get environment configuration for your project

$ firebase functions:config:get [path]

Example

const functions = require('firebase-functions')
console.log(functions.config().someservice.id)

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NOTE: You must redeploy functions to make the new configuration available.