GeezerVison

    
      const APP_ID: InjectionToken<string>;
    
    

A DI token representing a string ID, used primarily for prefixing application attributes and CSS styles when ViewEncapsulation#Emulated is being used.

The token is needed in cases when multiple applications are bootstrapped on a page (for example, using bootstrapApplication calls). In this case, ensure that those applications have different APP_ID value setup. For example:

bootstrapApplication(ComponentA, {
  providers: [
    { provide: APP_ID, useValue: 'app-a' },
    // ... other providers ...
  ]
});

bootstrapApplication(ComponentB, {
  providers: [
    { provide: APP_ID, useValue: 'app-b' },
    // ... other providers ...
  ]
});

By default, when there is only one application bootstrapped, you don't need to provide the APP_ID token (the ng will be used as an app ID).