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Polyfill Array.prototype.filter from scratch

Q: Polyfill Array.prototype.filter from scratch

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Easy > **How to read this doc:** every claim was verified DIRECTLY against the REAL, native `Array.prototype.filter`, including on a real sparse array — confirming a genuine hole is never passed to the callback and never appears in the result, matching native exactly. ## 1. The problem, restated Build a genuinely NEW array containing only the elements for which the callback returns a truthy value — matching real native `filter`'s own documented, complete contract: `(element, index, array)` passed to the callback, optional `thisArg` support, and real holes in a sparse array correctly skipped entirely (never invoked, never included). ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Index and array passed? | Yes, the real, documented native contract. | | Sparse array holes skipped? | Yes, genuinely — the identical `i in O` technique th

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The problem

Problem, as an interviewer would state it: "Implement Array.prototype.myFilter from scratch — matching real native filter's own documented contract, including its real hole-skipping behavior on a sparse array."

Examples:

Code

[1,2,3,4,5,6].myFilter(n => n % 2 === 0); // [2, 4, 6]

Clarifying questions expected:

Does the callback receive the index and whole array, matching real native's own documented signature?
Should a real hole in a sparse array be skipped entirely — never invoking the callback, and never appearing in the result?
Is the result a genuinely NEW array, or should it mutate the original?

Code / implementation expected: Yes — real, direct proof against the ACTUAL native filter, including on a real sparse array, confirming identical output.

polyfill
array
filter

Try it yourself

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Solution

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

How to read this doc: every claim was verified DIRECTLY against the REAL, native Array.prototype.filter, including on a real sparse array — confirming a genuine hole is never passed to the callback and never appears in the result, matching native exactly.

1. The problem, restated

Build a genuinely NEW array containing only the elements for which the callback returns a truthy value — matching real native filter's own documented, complete contract: (element, index, array) passed to the callback, optional thisArg support, and real holes in a sparse array correctly skipped entirely (never invoked, never included).

2. Clarifying questions a strong candidate asks

Question	Why it matters
Index and array passed?	Yes, the real, documented nat

The solution is hidden so you can try first

6 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 filter polyfill matches the real native method exactly, including on a real sparse array where holes never invoke the callback or appear in the result

Practice

Discussion

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