67 lines
1.9 KiB
JavaScript
67 lines
1.9 KiB
JavaScript
/**
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* Copyright (c) 2015-present, Facebook, Inc.
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*
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* This source code is licensed under the MIT license found in the
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* LICENSE file in the root directory of this source tree.
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*
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* @format
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* @preventMunge
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* @flow
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*/
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'use strict';
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/**
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* Checks whether a collection name (e.g. "Map" or "Set") has a native polyfill
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* that is safe to be used.
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*/
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function _shouldActuallyPolyfillES6Collection(collectionName: string): boolean {
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const Collection = global[collectionName];
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if (Collection == null) {
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return true;
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}
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// The iterator protocol depends on `Symbol.iterator`. If a collection is
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// implemented, but `Symbol` is not, it's going to break iteration because
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// we'll be using custom "@@iterator" instead, which is not implemented on
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// native collections.
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if (typeof global.Symbol !== 'function') {
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return true;
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}
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const proto = Collection.prototype;
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// These checks are adapted from es6-shim: https://fburl.com/34437854
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// NOTE: `isCallableWithoutNew` and `!supportsSubclassing` are not checked
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// because they make debugging with "break on exceptions" difficult.
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return (
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Collection == null ||
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typeof Collection !== 'function' ||
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typeof proto.clear !== 'function' ||
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new Collection().size !== 0 ||
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typeof proto.keys !== 'function' ||
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typeof proto.forEach !== 'function'
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);
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}
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const cache: {[name: string]: boolean} = {};
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/**
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* Checks whether a collection name (e.g. "Map" or "Set") has a native polyfill
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* that is safe to be used and caches this result.
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* Make sure to make a first call to this function before a corresponding
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* property on global was overriden in any way.
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*/
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function _shouldPolyfillES6Collection(collectionName: string) {
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let result = cache[collectionName];
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if (result !== undefined) {
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return result;
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}
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result = _shouldActuallyPolyfillES6Collection(collectionName);
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cache[collectionName] = result;
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return result;
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}
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module.exports = _shouldPolyfillES6Collection;
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