MediumDSAAsked in 20245 min read

Array subset check

Q: Array subset check

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the title gives no spec, so this doc defines two and checks both against brute force over 20,000 seeded random array pairs (0 mismatches). It also measures why the tempting one-liner is quadratic: at n = 20,000, nested `includes` took 318 ms versus 3.5 ms with a `Set`. ## 1. The problem, restated Decide whether the first array is contained in the second. Two reasonable definitions exist, and the interview is largely about noticing that: set containment asks only "is each value present?", while multiset containment asks "does the second array have enough copies of each value?". ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Do duplicates count? | Set semantics say `[1,1,1]` is a subset of `[1]`; multiset semantics say it is not. They need different code. | | Empty subset? | The empty array is a su

764 views

103 of 154

The problem

Problem, as an interviewer would state it: "Write isSubset(sub, sup) that returns whether every element of sub appears in sup. Discuss edge cases, runtime, and what subset should mean when there are duplicates."

Examples:

Code

isSubset([1, 2], [3, 2, 1]); // true
isSubset([1, 4], [1, 2, 3]); // false

Clarifying questions expected:

Set semantics (only presence matters) or multiset semantics (counts matter, so [1,1] is not a subset of [1])?
Is the empty array a subset of everything?
How are NaN, objects, and 1 versus "1" compared?

Code / implementation expected: Yes — real, direct proof against a brute-force reference, in both semantics.

js-coding
utility
algorithm

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: Medium

How to read this doc: the title gives no spec, so this doc defines two and checks both against brute force over 20,000 seeded random array pairs (0 mismatches). It also measures why the tempting one-liner is quadratic: at n = 20,000, nested includes took 318 ms versus 3.5 ms with a Set.

1. The problem, restated

Decide whether the first array is contained in the second. Two reasonable definitions exist, and the interview is largely about noticing that: set containment asks only "is each value present?", while multiset containment asks "does the second array have enough copies of each value?".

2. Clarifying questions a strong candidate asks

Question	Why it matters
Do duplicates count?	Set semantics say `[1,1,1]` is a subset of `[1]`; mul

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: set and multiset subset checks match brute force on 20,000 random pairs, NaN is handled, and a Set beats nested includes

Practice

Discussion

Other approaches, follow-ups and how it went in real interviews.

No comments yet. Share how you would answer it.

Question 103 of 154 in JavaScript coding

Keep your momentum going.

Mark this question solved to track your progress, then take the next one while it is fresh.

All JavaScript coding questions

Up nextMedium→

Implement Promise.allSettled()

Continue