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[BUGFIX] Only freeze empty array/dict with weakmap #361

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Jan 5, 2017
Merged

[BUGFIX] Only freeze empty array/dict with weakmap #361

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15 changes: 13 additions & 2 deletions packages/glimmer-runtime/lib/utils.ts
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Original file line number Diff line number Diff line change
@@ -1,7 +1,18 @@
import { Dict, dict } from 'glimmer-util';

export const EMPTY_ARRAY = Object.freeze([]);
export const EMPTY_DICT: Dict<any> = Object.freeze(dict<any>());
const HAS_NATIVE_WEAKMAP = (function() {
// detect if `WeakMap` is even present
let hasWeakMap = typeof WeakMap === 'function';
if (!hasWeakMap) { return false; }

let instance = new WeakMap();
// use `Object`'s `.toString` directly to prevent us from detecting
// polyfills as native weakmaps
return Object.prototype.toString.call(instance) === '[object WeakMap]';
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Unfortunately, core-js monkey patches toString to even emulate ^. Given that, it is unclear to me if we can detect the real thing any better then how you have written it.

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Confirm, I did a ton of research around this before settling for it in ember itself. I just don't think there is a better way that isn't more brittle than this check.

})();

export const EMPTY_ARRAY = HAS_NATIVE_WEAKMAP ? Object.freeze([]) : [];
export const EMPTY_DICT: Dict<any> = HAS_NATIVE_WEAKMAP ? Object.freeze(dict<any>()) : dict<any>();

export interface EnumerableCallback<T> {
(item: T): void;
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