/**
 * @license
 * Copyright 2019 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/**
 * Attempts to get the most accurate browser language setting.
 *
 * <p>Adapted from getUserLanguage in packages/auth/src/utils.js for TypeScript.
 *
 * <p>Defers default language specification to server logic for consistency.
 *
 * @param navigatorLanguage Enables tests to override read-only {@link NavigatorLanguage}.
 */
export function getUserLanguage(
  navigatorLanguage: NavigatorLanguage = navigator
): string {
  return (
    // Most reliable, but only supported in Chrome/Firefox.
    (navigatorLanguage.languages && navigatorLanguage.languages[0]) ||
    // Supported in most browsers, but returns the language of the browser
    // UI, not the language set in browser settings.
    navigatorLanguage.language
    // Polyfill otherwise.
  );
}