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Query String Generator

Q: Query String Generator

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** every encoding claim below was verified against the REAL, built-in `encodeURIComponent` — nothing here reimplements percent-encoding from scratch, since that would be reinventing a real, already-correct browser/Node primitive. ## 1. The problem, restated `toQueryString(obj)` converts a plain object's entries into a real `key=value&key2=value2`-style string: array values become multiple repeated `key=element` pairs, and `null`/`undefined` values are skipped entirely (not included as an empty or literal "undefined" string). ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Array values: repeated keys or comma-joined? | Repeated keys (`tags=a&tags=b`) is the real, more standard convention most servers correctly parse; comma-joining is a real, valid alternative some APIs expect instead. | | Skip null/un

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76 of 154

The problem

Problem, as an interviewer would state it: "Write toQueryString(obj) — convert a plain object into a URL query string, correctly percent-encoding values, handling array values as repeated keys, and skipping null/undefined."

Examples:

Code

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

Clarifying questions expected:

Should array values become repeated key=val pairs, or a single comma-joined value?
Should null/undefined values be skipped entirely, or included as an empty string?
Does the key itself ever need encoding too, not just the value?

Code / implementation expected: Yes — real, direct proof handling a string with a space, an array value, a number, and an undefined field, verified against real encodeURIComponent output.

object
string

Try it yourself

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Solution

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

How to read this doc: every encoding claim below was verified against the REAL, built-in encodeURIComponent — nothing here reimplements percent-encoding from scratch, since that would be reinventing a real, already-correct browser/Node primitive.

1. The problem, restated

toQueryString(obj) converts a plain object's entries into a real key=value&key2=value2-style string: array values become multiple repeated key=element pairs, and null/undefined values are skipped entirely (not included as an empty or literal "undefined" string).

2. Clarifying questions a strong candidate asks

Question	Why it matters
Array values: repeated keys or comma-joined?	Repeated keys (`tags=a&tags=b`) is the real, more standard convention most servers co

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: toQueryString() correctly percent-encodes a space, fans out an array into repeated keys, and skips an undefined field

Practice

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