react-native/Libraries/vendor/core/_shouldPolyfillES6Collectio...

64 lines
2.0 KiB
JavaScript

/**
* Copyright (c) 2015-present, Facebook, Inc.
*
* This source code is licensed under the MIT license found in the
* LICENSE file in the root directory of this source tree.
*
* @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;