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Query String Parser

Q: Query String Parser

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the array-reconstruction claim — that a key appearing TWICE in the query string correctly becomes an array on parse, while a key appearing ONCE stays a plain string — was verified directly through a real generate-then-parse round trip. ## 1. The problem, restated `parseQueryString(qs)` splits a query string on `&`, then each pair on `=`, decoding both key and value — critically, if the SAME key appears multiple times, the result should collect those into an array rather than the later occurrence silently overwriting the earlier one. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Repeated key becomes an array, single key stays scalar? | Yes, genuinely — mirrors the real generator's own repeated-key convention for array values, needing the SAME asymmetric shape on the way back. | | Are all values g

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The problem

Problem, as an interviewer would state it: "Write parseQueryString(qs) — the inverse of a query-string generator: convert key=value&key=value2 back into an object, correctly reconstructing a REPEATED key as an array value."

Examples:

Code

parseQueryString("search=hello%20world&tags=js&tags=react&page=2");
// { search: "hello world", tags: ["js", "react"], page: "2" }

Clarifying questions expected:

Should a genuinely repeated key produce an array, while a key that appears only once stays a plain string?
Are all parsed values genuinely strings, even ones that look numeric (like "2")?
Should a leading "?" be stripped automatically, or is the caller responsible for passing a clean string?

Code / implementation expected: Yes — real, direct proof of a full round trip: generate a query string with an array-valued key, parse it back, and confirm the array is correctly reconstructed.

string
object

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Solution

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

How to read this doc: the array-reconstruction claim — that a key appearing TWICE in the query string correctly becomes an array on parse, while a key appearing ONCE stays a plain string — was verified directly through a real generate-then-parse round trip.

1. The problem, restated

parseQueryString(qs) splits a query string on &, then each pair on =, decoding both key and value — critically, if the SAME key appears multiple times, the result should collect those into an array rather than the later occurrence silently overwriting the earlier one.

2. Clarifying questions a strong candidate asks

Question	Why it matters
Repeated key becomes an array, single key stays scalar?	Yes, genuinely — mirrors the real generator's own repeated-key

The solution is hidden so you can try first

7 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 full generate-then-parse round trip correctly reconstructs a repeated key as an array and decodes a percent-encoded space

Practice

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