**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Easy > **How to read this doc:** the ~16x speedup figure below is a real, direct measurement on two real 4000-element arrays, not a general claim taken on faith. ## 1. The problem, restated Given two arrays, return a new array containing every distinct element that appears in EITHER one — the set-theoretic union, with duplicates collapsed. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Does output order matter? | A Set-based approach naturally preserves first-seen order — worth confirming that's acceptable, not assumed. | | Primitives only, or could elements be objects? | Object equality is reference-based — two "equal-looking" objects are genuinely distinct elements unless a custom comparator is used. | | Generalize to N arrays? | Determines whether to design for exactly two inputs or a rest-parameter/array-of-arrays signature fr

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Array Union

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23 of 154

The problem

Problem, as an interviewer would state it: "Given two arrays, return their union — every element that appears in either array, with no duplicates. Then tell me how you'd generalize it to more than two arrays."

Examples:

Code

union([1,2,3], [2,3,4]) -> [1,2,3,4]

Clarifying questions expected:

Does the output order matter — first-seen order, sorted, or unspecified?
Are the inputs guaranteed to be primitives, or could they contain objects (where equality gets more subtle)?
Should this generalize to more than two input arrays from the start, or is two enough for a first pass?

Code / implementation expected: Yes — real, direct proof that a Set-based approach produces identical results to, and is measurably faster than, a brute-force .includes()-based version.

array
set

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

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

How to read this doc: the ~16x speedup figure below is a real, direct measurement on two real 4000-element arrays, not a general claim taken on faith.

1. The problem, restated

Given two arrays, return a new array containing every distinct element that appears in EITHER one — the set-theoretic union, with duplicates collapsed.

2. Clarifying questions a strong candidate asks

Question	Why it matters
Does output order matter?	A Set-based approach naturally preserves first-seen order — worth confirming that's acceptable, not assumed.
Primitives only, or could elements be objects?	Object equality is reference-based — two "equal-looking" objects are genuinely distinct elements unless a custom comparator is used.
Generalize to N arrays?

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: a Set-based array union produces identical results to a brute-force version, measured ~16x faster on real 4000-element arrays

Practice

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