skills/firebase-app-hosting-basics/references/emulation.md

# 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.

```yaml
# 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:

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

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

```bash
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:

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

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