Use WeakMap and FinalizationRegistry when available. #132
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benjamn
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Jan 19, 2021
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| if (useWeakKeys && key && typeof key === "object") { | ||
| let ref = weakRefs!.get(key)!; | ||
| if (!ref) { | ||
| weakRefs!.set(key, ref = {}); | ||
| registry!.register(key, ref); |
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The ! coercions here are safe because useWeakKeys implies weakRefs and registry must be defined.
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| // Optional WeakMap mapping object keys returned by makeCacheKey to | ||
| // empty object references that will be stored in the cache instead of | ||
| // the original key object. Undefined/unused if useWeakKeys is false. | ||
| // It's tempting to use WeakRef objects instead of empty objects, but | ||
| // we never actually need to call .deref(), and using WeakRef here | ||
| // noticeably slows down cache performance. | ||
| const weakRefs = useWeakKeys | ||
| ? new WeakMap<object, {}>() |
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Realizing that WeakRef is not necessary was the biggest (pleasant) surprise of this work.
benjamn
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Jan 19, 2021
| "lib": ["es2015"], | ||
| "lib": ["esnext"], |
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This change brings in the TypeScript typings for WeakRef and FinalizationRegistry.
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Although
WeakRefandFinalizationRegistryare not yet supported by all browsers, I was pleasantly surprised to learn that more than 68% of browsers already have support for these new APIs.Using a
FinalizationRegistryis appealing for this library, because it allows deleting keys from the internal LRU cache when they become unreachable, which makes sense because an unreachable key can never be looked up in the cache again. Without this functionality, a naive LRU cache will keep keys alive that could otherwise be garbage collected.This PR uses a
FinalizationRegistrywhen possible, but also allows the behavior to be configured with theuseWeakKeysoption for thewrapfunction, since the additional overhead of the registration and cleanup has a noticeable performance cost (the performance stress test run bynpm testnow takes ~1325ms instead of ~950ms, on my machine).