Implement a WeakMap-Like API Shape (and Why True Weak References Cannot Be Polyfilled)

Q: Implement a WeakMap-Like API Shape (and Why True Weak References Cannot Be Polyfilled)

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the honest, real limitation this question specifically tests understanding of — that a WeakMap polyfill can match the API but can NEVER replicate true weak references — was verified directly by measuring that the polyfill's own internal storage size stayed completely unchanged even after the only external reference to a tracked key was dropped. ## 1. The problem, restated A real `WeakMap` lets an object be used as a key WITHOUT preventing that object from being garbage collected once nothing else references it — the map entry itself disappears automatically. JavaScript deliberately gives NO API for observing or controlling garbage collection directly, which means a polyfill built from ordinary JS constructs (a plain `Map`, an array, anything) can match the surface API, but structurally CANNOT replicate the real weak-reference behavior — it will alwa

Custom WeakMap reference tracker | JS Coding Challenge

The problem

Problem, as an interviewer would state it: "Implement an object exposing the same API SHAPE as a real WeakMap (get/set/has/delete) — then explain, honestly, why it can NEVER actually replicate WeakMap's real, defining behavior: automatic garbage collection of unreferenced keys."

Examples:

Code

const wm = new FakeWeakMap();
wm.set(someObject, "data");
// the API works identically -- but the polyfill can NEVER let someObject be garbage collected

Clarifying questions expected:

Should the polyfill genuinely try to replicate weak references, or is matching the API SHAPE while being honest about the limitation the actual goal?
Should it reject non-object keys, matching real WeakMap's own documented contract?
Is there ANY real JS-level way to observe or influence garbage collection directly?

Code / implementation expected: Yes — real, direct proof that the API shape works correctly, PLUS a real, honest demonstration of the actual memory-leak limitation: the polyfill's internal storage is proven to retain a reference even after every other reference to a key is dropped.

js-coding
utility
algorithm

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

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Solution

Target Audience: Engineers preparing for practical JavaScript coding interviews. Difficulty: Medium

How to read this doc: the honest, real limitation this question specifically tests understanding of — that a WeakMap polyfill can match the API but can NEVER replicate true weak references — was verified directly by measuring that the polyfill's own internal storage size stayed completely unchanged even after the only external reference to a tracked key was dropped.

1. The problem, restated

A real WeakMap lets an object be used as a key WITHOUT preventing that object from being garbage collected once nothing else references it — the map entry itself disappears automatically. JavaScript deliberately gives NO API for observing or controlling garbage collection directly, which means a polyfill built from ordinary JS constructs (a plain Map, an array, anything) can matc

The solution is hidden so you can try first

7 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal, direct proof: the polyfill's API shape works correctly, but its internal storage genuinely never shrinks after the only external key reference is dropped