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JavaScript·Advanced·Coding·1 min read

Why do WeakMap keys not prevent collection?

Short interview answer

A WeakMap does not keep its object keys strongly reachable. When a key has no other strong path from a root, its entry may disappear. Weak collections are non-enumerable because collection timing cannot become observable.

Example

JavaScript
const meta = new WeakMap();let node = document.createElement('div');meta.set(node, { renderedAt: Date.now() });
node = null; // nothing else references the div ->             // the div AND its WeakMap entry become collectible

Key takeaway

Explain the underlying mental model clearly, then support it with a concrete example and its trade-offs.

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