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Implement difference(a, b) — elements in a not in b

Q: Implement difference(a, b) — elements in a not in b

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the real, defining ASYMMETRY this question tests — that `difference` returns elements in `a` but NOT `b`, specifically NOT a symmetric set difference — was verified directly, alongside confirming array `a`'s own original relative order is genuinely preserved in the result, not sorted or reordered. ## 1. The problem, restated Return every element of array `a` that does NOT appear anywhere in array `b` — a genuinely ASYMMETRIC operation (elements unique to `a`, specifically, not a general "items in either but not both") — preserving `a`'s own original relative order in the result, matching lodash's own well-known `_.difference` contract. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Asymmetric, specifically a-minus-b? | Yes, genuinely — real `_.difference` is NOT a symmetric set difference; it spe

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The problem

Problem, as an interviewer would state it: "Implement difference(a, b) — returning every element of a that does NOT appear anywhere in b, preserving a's own original relative order, matching lodash's own well-known _.difference behavior."

Examples:

Code

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

Clarifying questions expected:

Is this a genuinely ASYMMETRIC operation — elements in a but not b, as opposed to a full, symmetric set difference (in either but not both)?
Should the result preserve a's own original relative order?
What real, efficient lookup structure should be used to check membership in b, avoiding a naive nested-loop approach?

Code / implementation expected: Yes — real, direct proof of basic asymmetric difference, confirmation a's own order is preserved, and confirmation of the correct, real "b contains everything" edge case.

set
difference
array

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 real, defining ASYMMETRY this question tests — that difference returns elements in a but NOT b, specifically NOT a symmetric set difference — was verified directly, alongside confirming array a's own original relative order is genuinely preserved in the result, not sorted or reordered.

1. The problem, restated

Return every element of array a that does NOT appear anywhere in array b — a genuinely ASYMMETRIC operation (elements unique to a, specifically, not a general "items in either but not both") — preserving a's own original relative order in the result, matching lodash's own well-known _.difference contract.

2. Clarifying questions a strong candidate asks

Question	Why it matters
Asymmetric, spec

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: difference() correctly returns elements unique to a, preserving a's own original relative order via a Set-based O(1) membership check against b

Practice

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