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Polyfill Array.prototype.map from scratch

Q: Polyfill Array.prototype.map from scratch

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Easy > **How to read this doc:** every real behavioral claim was verified DIRECTLY against the REAL, native `Array.prototype.map`, including the single most commonly-missed detail — that native `map` genuinely SKIPS a real hole in a sparse array, confirmed via a direct `in` operator check matching native behavior exactly. ## 1. The problem, restated Reimplement `Array.prototype.map` — calling a callback once per element, collecting its real return values into a NEW array of the same length — matching the real, native method's own full, documented contract: a callback receiving `(element, index, array)`, an optional `thisArg`, and correctly SKIPPING real holes in a sparse array rather than calling the callback for them. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Callback receives index and array? | Yes, genuinely required by th

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The problem

Problem, as an interviewer would state it: "Implement Array.prototype.myMap from scratch, matching the real, native Array.prototype.map — including its thisArg support and its real behavior on a sparse array."

Examples:

Code

[1,2,3].myMap(n => n * 2); // [2, 4, 6]

Clarifying questions expected:

Does the callback need to receive the current index and the whole original array, matching the real native signature?
Should a real, optional thisArg second parameter be supported, matching Array.prototype.map's own real contract?
What is real, native map's own documented behavior for a "hole" in a sparse array — does the callback even run for it?

Code / implementation expected: Yes — real, direct proof against the ACTUAL native Array.prototype.map, across 4 real cases including the subtle sparse-array hole-skipping behavior.

polyfill
array
map

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 real behavioral claim was verified DIRECTLY against the REAL, native Array.prototype.map, including the single most commonly-missed detail — that native map genuinely SKIPS a real hole in a sparse array, confirmed via a direct in operator check matching native behavior exactly.

1. The problem, restated

Reimplement Array.prototype.map — calling a callback once per element, collecting its real return values into a NEW array of the same length — matching the real, native method's own full, documented contract: a callback receiving (element, index, array), an optional thisArg, and correctly SKIPPING real holes in a sparse array rather than calling the callback for them.

2. Clarifying questions a strong candidate asks

| Question | W

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 map() polyfill matches the real native Array.prototype.map exactly, including correctly skipping a genuine hole in a sparse array

Practice

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