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Implement _.compact — remove falsy values

Q: Implement _.compact — remove falsy values

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** while genuinely simple to implement, this question's real value is in correctly, completely naming EVERY JavaScript falsy value — verified directly against a real array containing all six of them at once (`0`, `false`, `""`, `null`, `undefined`, `NaN`), confirming every single one is correctly removed. ## 1. The problem, restated Return a genuinely NEW array containing only the TRUTHY elements of the input, removing every FALSY one — JavaScript defines exactly SIX real falsy values: `false`, `0` (and `-0`), `""` (empty string), `null`, `undefined`, and `NaN` — matching lodash's own well-known `_.compact` contract. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Complete definition of falsy? | Exactly SIX real values in JavaScript — worth naming ALL of them explicitly, since missing even one in a m

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The problem

Problem, as an interviewer would state it: "Implement compact(array) — removing every FALSY value from the array, matching lodash's own well-known _.compact behavior."

Examples:

Code

compact([0, 1, false, 2, "", 3]); // [1, 2, 3]

Clarifying questions expected:

What is the real, complete, correct definition of "falsy" in JavaScript — every value that would evaluate as false in a boolean context?
Does this need to preserve the relative order of the surviving truthy elements?
Is this genuinely just a one-line filter, or is there a deeper real correctness concern worth discussing?

Code / implementation expected: Yes — real, direct proof against an array containing EVERY genuine JavaScript falsy value, confirming all are correctly removed while truthy order is preserved.

array
compact
falsy

Try it yourself

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Solution

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

How to read this doc: while genuinely simple to implement, this question's real value is in correctly, completely naming EVERY JavaScript falsy value — verified directly against a real array containing all six of them at once (0, false, "", null, undefined, NaN), confirming every single one is correctly removed.

1. The problem, restated

Return a genuinely NEW array containing only the TRUTHY elements of the input, removing every FALSY one — JavaScript defines exactly SIX real falsy values: false, 0 (and -0), "" (empty string), null, undefined, and NaN — matching lodash's own well-known _.compact contract.

2. Clarifying questions a strong candidate asks

Question	Why it matters
Complete definition of falsy?	Exactl

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: compact() correctly removes every one of JavaScript's six real falsy values, verified against a single array containing all of them at once

Practice

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