text: /**
   * Copyright (c) 2014-present, Facebook, Inc. All rights reserved.
   *
   * This source code is licensed under the BSD-style license found in the
   * LICENSE file in the root directory of this source tree. An additional grant
   * of patent rights can be found in the PATENTS file in the same directory.
   *
   * @flow
   */
  
  const SUPPORTED_PLATFORM_EXTS = {
    android: true,
    ios: true,
    native: true,
    web: true,
  };
  
  // Extract platform extension: index.ios.js -> ios
  function getPlatformExtension(
    file: string,
    platforms?: Array<string>,
  ): ?string {
    const last = file.lastIndexOf('.');
    const secondToLast = file.lastIndexOf('.', last - 1);
    if (secondToLast === -1) {
      return null;
    }
    const platform = file.substring(secondToLast + 1, last);
    // If an overriding platform array is passed, check that first
  
    if (platforms && platforms.indexOf(platform) !== -1) {
      return platform;
    }
    return SUPPORTED_PLATFORM_EXTS[platform] ? platform : null;
  }
  
  module.exports = getPlatformExtension;
  