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Deep Map Keys

Q: Deep Map Keys

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** beyond the happy path, this doc verifies a real, easy-to-miss hazard directly: if the mapper function returns the string `"__proto__"`, a plain `out[newKey] = ...` assignment silently swallows the entry instead of creating a property. A defineProperty-based version was verified to keep it as a normal own key. ## 1. The problem, restated Return a copy of a nested structure in which every plain-object key has been replaced by `fn(key)`, at every depth and inside arrays, with values (and non-plain objects) left exactly as they were. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Arrays of objects? | Real API payloads are full of them. An array itself has no key to rename, but its object elements do, so map over the array and recurse. | | Key collisions? | `{ a: 1, A: 2 }` with a lowercasing mapper p

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The problem

Problem, as an interviewer would state it: "Implement deepMapKeys(value, fn) — recursively transforming every KEY of nested objects (including objects inside arrays) with fn, leaving values untouched."

Examples:

Code

deepMapKeys({ a: { b: [{ c: 1 }] } }, k => k.toUpperCase());
// { A: { B: [{ C: 1 }] } }

Clarifying questions expected:

Should keys inside objects that live inside arrays be mapped too?
If two keys map to the same new key, which one wins?
Which values count as "objects to descend into" — and what about a Date or a circular reference?

Code / implementation expected: Yes — real, direct proof at every depth, on a collision, and on a circular structure.

object
recursion

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: beyond the happy path, this doc verifies a real, easy-to-miss hazard directly: if the mapper function returns the string "__proto__", a plain out[newKey] = ... assignment silently swallows the entry instead of creating a property. A defineProperty-based version was verified to keep it as a normal own key.

1. The problem, restated

Return a copy of a nested structure in which every plain-object key has been replaced by fn(key), at every depth and inside arrays, with values (and non-plain objects) left exactly as they were.

2. Clarifying questions a strong candidate asks

Question	Why it matters
Arrays of objects?	Real API payloads are full of them. An array itself has no key to rename, but its object elements do, s

The solution is hidden so you can try first

5 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: keys are mapped at every depth including inside arrays, and a mapper returning __proto__ shows why defineProperty is the safe write

Practice

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