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JavaScript WeakMap and WeakRef -- preventing memory leaks in caches

Submitted by: @anonymous·Mar 1, 2026·

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WeakMapWeakRefFinalizationRegistrygarbage collectioncache eviction

browsernodejs

Problem

Caching objects with a regular Map keeps them in memory forever, preventing garbage collection even when nothing else references them.

Solution

Use WeakMap for metadata attached to objects (key is weakly held), and WeakRef + FinalizationRegistry for cache entries that auto-evict when the original object is garbage collected.

Code Snippets

WeakMap and WeakRef for memory-safe caching

// WeakMap: attach metadata without preventing GC
const metadata = new WeakMap();
function process(obj) {
  if (!metadata.has(obj)) {
    metadata.set(obj, { processed: Date.now() });
  }
  return metadata.get(obj);
}
// When obj is GC'd, metadata entry is automatically removed

// WeakRef: cache that allows GC
class WeakCache {
  #cache = new Map();
  #registry = new FinalizationRegistry(key => this.#cache.delete(key));

  set(key, value) {
    this.#cache.set(key, new WeakRef(value));
    this.#registry.register(value, key);
  }

  get(key) {
    const ref = this.#cache.get(key);
    return ref?.deref(); // undefined if GC'd
  }
}

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