/**
 * Copyright (c) 2015-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.
 *
 * @providesModule _shouldPolyfillES6Collection
 * @preventMunge
 * @flow
 */
'use strict';

/**
 * Checks whether a collection name (e.g. "Map" or "Set") has a native polyfill
 * that is safe to be used.
 */
function _shouldActuallyPolyfillES6Collection(collectionName: string): boolean {
  var Collection = global[collectionName];
  if (Collection == null) {
    return true;
  }

  // The iterator protocol depends on `Symbol.iterator`. If a collection is
  // implemented, but `Symbol` is not, it's going to break iteration because
  // we'll be using custom "@@iterator" instead, which is not implemented on
  // native collections.
  if (typeof global.Symbol !== 'function') {
    return true;
  }

  var proto = Collection.prototype;

  // These checks are adapted from es6-shim: https://fburl.com/34437854
  // NOTE: `isCallableWithoutNew` and `!supportsSubclassing` are not checked
  // because they make debugging with "break on exceptions" difficult.
  return Collection == null ||
    typeof Collection !== 'function' ||
    typeof proto.clear !== 'function' ||
    new Collection().size !== 0 ||
    typeof proto.keys !== 'function' ||
    typeof proto.forEach !== 'function';
}

const cache: { [name: string]: bool } = {};

  /**
   * Checks whether a collection name (e.g. "Map" or "Set") has a native polyfill
   * that is safe to be used and caches this result.
   * Make sure to make a first call to this function before a corresponding
   * property on global was overriden in any way.
   */
function _shouldPolyfillES6Collection(collectionName: string) {
    let result = cache[collectionName];
    if (result !== undefined) {
      return result;
    }

    result = _shouldActuallyPolyfillES6Collection(collectionName);
    cache[collectionName] = result;
    return result;
}

module.exports = _shouldPolyfillES6Collection;