/firebase-app-hosting-basics

App Hosting Emulation

You can test your App Hosting setup locally using the Firebase Local Emulator Suite. This allows you to verify your app's behavior with environment variables and secrets before deploying.

Configuration: apphosting.emulator.yaml

This optional file overrides apphosting.yaml settings specifically for the local emulator. Use it to provide local secret values or override resource configs. If it contains sensitive values such as API keys, do not commit it to source control.

# apphosting.emulator.yaml (gitignored usually)
runConfig:
  cpu: 1
  memoryMiB: 512

env:
  - variable: API_KEY
    value: "local-dev-api-key" # Override secret with local value

Running the Emulator

To start the App Hosting emulator:

npx -y firebase-tools@latest emulators:start --only apphosting

Or, if you are also using other emulators (Auth, Firestore, etc.):

npx -y firebase-tools@latest emulators:start

Capabilities

• Builds your app: Runs the build command defined in your package.json to generate the serving artifact.
• Serves locally: Runs the app on localhost:5004 (default). Configurable by setting host and port in the emulators block of firebase.json, like so:

{
  "emulators": {
    "apphosting": {
      "host": "localhost",
      "port": 5004
    }
  }
}

• Env Var Injection: Injects variables defined in apphosting.yaml and apphosting.emulator.yaml into the process.