getApp
Retrieves a {@link @firebase/app#FirebaseApp} instance.
When called with no arguments, the default app is returned. When an app name is provided, the app corresponding to that name is returned.
An exception is thrown if the app being retrieved has not yet been initialized.
@example
// Return the default app
const app = getApp();
@example
// Return a named app
const otherApp = getApp("otherApp");