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Implement Array.prototype.flat() — flatten with depth

Q: Implement Array.prototype.flat() — flatten with depth

**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.flat`, at three different depth arguments (the default, an explicit 2, and Infinity), confirming identical output in every case, including on a real sparse array. ## 1. The problem, restated Flatten nested arrays into a single array, recursing up to `depth` levels deep — matching real native `flat`'s own documented default of `depth = 1`, its support for a genuinely unlimited `Infinity` depth, and its real, additional behavior of removing sparse-array holes at every level it flattens through. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Default depth? | `1` — real native `flat`'s own documented default, flattening only the OUTERMOST level of nesting unless told otherwise. | | Infinity valid? | Yes — a real, genuinely comm

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The problem

Problem, as an interviewer would state it: "Implement Array.prototype.myFlat(depth) from scratch — recursively flattening nested arrays up to the given depth, matching real native flat's own documented contract, including its default depth of 1."

Examples:

Code

[1, [2, 3], [4, [5, 6]]].myFlat();  // [1, 2, 3, 4, [5, 6]] -- default depth 1
[1, [2, 3], [4, [5, 6]]].myFlat(2); // [1, 2, 3, 4, 5, 6]

Clarifying questions expected:

What is the real, correct default depth when no argument is passed?
Should Infinity be a genuinely valid depth argument, flattening completely regardless of nesting?
Does flat() also remove real holes in a sparse array, at every flattened level?

Code / implementation expected: Yes — real, direct proof against the ACTUAL native flat, at the default depth, an explicit depth of 2, and Infinity, confirming identical output in every case.

array
flat
recursion

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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.flat, at three different depth arguments (the default, an explicit 2, and Infinity), confirming identical output in every case, including on a real sparse array.

1. The problem, restated

Flatten nested arrays into a single array, recursing up to depth levels deep — matching real native flat's own documented default of depth = 1, its support for a genuinely unlimited Infinity depth, and its real, additional behavior of removing sparse-array holes at every level it flattens through.

2. Clarifying questions a strong candidate asks

Question	Why it matters
Default depth?	`1` — real native `flat`'s own documented default, flattening only the

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 flat polyfill matches real native flat at the default depth, an explicit depth of 2, and Infinity, including hole-removal on a sparse array

Practice

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