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Custom Array.prototype.flatmap polyfill

Q: Custom Array.prototype.flatmap polyfill

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Easy > **How to read this doc:** the real, defining constraint this question tests — that `flatMap` flattens EXACTLY one level, never more, unlike `flat`'s own configurable depth argument — was verified directly: a callback returning a doubly-nested array (`[[n]]`) correctly left the INNER array still nested in the result, matching real native `flatMap` exactly. ## 1. The problem, restated For each element, invoke a callback and collect its result — if the result is itself a real array, SPREAD it (flattening exactly one level) into the final result; otherwise, push it directly — matching real native `flatMap`'s own fixed, non-configurable one-level flattening contract, genuinely distinct from `flat`'s own arbitrary-depth support. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Fixed at exactly 1 level? | Yes, genuinely — real nativ

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18 of 154

The problem

Problem, as an interviewer would state it: "Implement Array.prototype.myFlatMap from scratch — mapping each element and then flattening the result by exactly ONE level, matching real native flatMap's own fixed-depth contract."

Examples:

Code

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

Clarifying questions expected:

Is the flattening depth for flatMap genuinely fixed at exactly 1 level, with no way to configure it deeper?
If the callback returns a non-array value for some elements, is that value simply pushed directly, without error?
Is flatMap genuinely equivalent to calling map() then flat(1) separately, or is there a real, meaningful difference?

Code / implementation expected: Yes — real, direct proof against the ACTUAL native flatMap, including a case with a doubly-nested result, confirming the ONE-level-only flattening behavior exactly.

js-coding
utility
algorithm

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Solution

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

How to read this doc: the real, defining constraint this question tests — that flatMap flattens EXACTLY one level, never more, unlike flat's own configurable depth argument — was verified directly: a callback returning a doubly-nested array ([[n]]) correctly left the INNER array still nested in the result, matching real native flatMap exactly.

1. The problem, restated

For each element, invoke a callback and collect its result — if the result is itself a real array, SPREAD it (flattening exactly one level) into the final result; otherwise, push it directly — matching real native flatMap's own fixed, non-configurable one-level flattening contract, genuinely distinct from flat's own arbitrary-depth support.

2. Clarifying questions a strong candidate asks

| Q

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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 flatMap polyfill matches real native flatMap exactly, including that it flattens only one level deep, unlike flat(Infinity)

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