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Clear Nullish Values

Q: Clear Nullish Values

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the spec is deliberately ambiguous about arrays ("preserving array index structures"), so this doc verifies BOTH readings directly: a compact mode that removes nullish elements and shifts the rest down, and a preserve mode that leaves real holes so survivors keep their exact indices. ## 1. The problem, restated Walk an arbitrarily nested structure of plain objects and arrays and drop every `null` / `undefined` — while leaving every other value (including the falsy `0`, `""`, `false`) alone, returning a NEW structure, and surviving a cycle instead of recursing forever. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Only nullish, or all falsy? | Nullish means exactly `null` and `undefined` (the `== null` check). Dropping `0` or `false` would silently destroy real data. | | Array semantics? | Removi

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The problem

Problem, as an interviewer would state it: "Implement clearNullish(value) — recursively removing every null and undefined from an object or array, without touching other falsy values, without mutating the input, and without crashing on a circular reference."

Examples:

Code

clearNullish({ a: 1, b: null, c: { d: undefined, e: 0 } });
// { a: 1, c: { e: 0 } }

Clarifying questions expected:

Should 0, "" and false survive? (They are falsy but not nullish.)
For arrays, should nullish elements be removed and the rest shifted down, or should surviving elements keep their original indices?
What should happen with a circular reference, and with non-plain objects such as a Date?

Code / implementation expected: Yes — real, direct proof on nested data, on a circular structure, and in both array modes.

object
recursion

Try it yourself

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Solution

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

How to read this doc: the spec is deliberately ambiguous about arrays ("preserving array index structures"), so this doc verifies BOTH readings directly: a compact mode that removes nullish elements and shifts the rest down, and a preserve mode that leaves real holes so survivors keep their exact indices.

1. The problem, restated

Walk an arbitrarily nested structure of plain objects and arrays and drop every null / undefined — while leaving every other value (including the falsy 0, "", false) alone, returning a NEW structure, and surviving a cycle instead of recursing forever.

2. Clarifying questions a strong candidate asks

Question	Why it matters
Only nullish, or all falsy?	Nullish means exactly `null` and `undefined` (the `== n

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: nullish values are removed recursively, other falsy values survive, a cycle is preserved, and arrays support compact and index-preserving modes

Practice

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