How to use the google-auth-library.GCPEnv.APP_ENGINE function in google-auth-library

export async function getDefaultResource(auth: GoogleAuth) {
  const env = await auth.getEnv();

  switch (env) {
    case GCPEnv.KUBERNETES_ENGINE:
      return getGKEDescriptor().catch(() => getGlobalDescriptor());
    case GCPEnv.APP_ENGINE:
      return getGAEDescriptor().catch(() => getGlobalDescriptor());
    case GCPEnv.CLOUD_FUNCTIONS:
      return getCloudFunctionDescriptor();
    case GCPEnv.COMPUTE_ENGINE:
      // Test for compute engine should be done after all the rest -
      // everything runs on top of compute engine.
      return getGCEDescriptor().catch(() => getGlobalDescriptor());
    default:
      return getGlobalDescriptor();
  }
}

async getEnv() {
              return GCPEnv.APP_ENGINE;
            },
          };