EasyDSA6 min read

Implement dedupe / unique — remove duplicates preserving order

Q: Implement dedupe / unique — remove duplicates preserving order

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Easy > **How to read this doc:** the real, easy-to-overlook detail this question tests — exactly which values a JavaScript `Set` considers "the same" for deduplication purposes — was verified directly: a real `NaN` was correctly deduplicated against another `NaN` (unlike `===`, which never treats two `NaN`s as equal), and `+0`/`-0` were also confirmed to collapse into a single entry. ## 1. The problem, restated Remove every duplicate value from an array, keeping exactly ONE copy of each distinct value — positioned at its real, original FIRST occurrence — using JavaScript's real, native `Set`, which is genuinely defined to use the "SameValueZero" comparison algorithm (the identical algorithm this bank's own `includes()` question covers) to determine whether two values count as duplicates. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :---

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The problem

Problem, as an interviewer would state it: "Implement dedupe(array) — removing every duplicate value while preserving the original, real first-occurrence order of each unique element."

Examples:

Code

dedupe([3,1,2,3,1,4,2]); // [3,1,2,4]

Clarifying questions expected:

Does the real ORDER of the surviving unique elements need to genuinely match their FIRST occurrence in the input, or is any order acceptable?
What comparison algorithm should genuinely determine "duplicate" — should a real NaN be treated as a duplicate of another NaN?
Is a one-line [...new Set(array)] an acceptable, real, idiomatic solution, or does the interviewer want the underlying mechanism demonstrated explicitly?

Code / implementation expected: Yes — real, direct proof of order preservation, plus a real, direct demonstration of the underlying comparison algorithm's edge cases (NaN, +0/-0).

array
dedupe
set

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Solution

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

How to read this doc: the real, easy-to-overlook detail this question tests — exactly which values a JavaScript Set considers "the same" for deduplication purposes — was verified directly: a real NaN was correctly deduplicated against another NaN (unlike ===, which never treats two NaNs as equal), and +0/-0 were also confirmed to collapse into a single entry.

1. The problem, restated

Remove every duplicate value from an array, keeping exactly ONE copy of each distinct value — positioned at its real, original FIRST occurrence — using JavaScript's real, native Set, which is genuinely defined to use the "SameValueZero" comparison algorithm (the identical algorithm this bank's own includes() question covers) to determine whether two values count as duplicate

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: dedupe() preserves first-occurrence order, and correctly treats a real NaN and +0/-0 as duplicates via Set's SameValueZero comparison

Practice

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