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Custom getElementsByClassName traverse

Q: Custom getElementsByClassName traverse

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** correctness was verified the strongest possible way — by comparing the custom implementation output DIRECTLY against the REAL, native `getElementsByClassName` on the identical real DOM tree, not just checking it against a hand-written expected list. ## 1. The problem, restated Recursively walk a DOM subtree starting from a given root, collecting every descendant element whose `class` attribute includes the target class name — matching the real, native `getElementsByClassName`'s own documented behavior (including elements with MULTIPLE classes, one of which matches). ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Multiple classes on one element? | Yes, genuinely required — `classList.contains` (or a real, careful split-and-check) correctly matches regardless of how many OTHER classes an element al

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73 of 154

The problem

Problem, as an interviewer would state it: "Implement a custom getElementsByClassName(root, className) from scratch, using a recursive tree traversal instead of the built-in DOM method."

Examples:

Code

customGetElementsByClassName(document.body, "box"); // an array of every descendant with class "box"

Clarifying questions expected:

Should this match an element with MULTIPLE classes, as long as one of them equals the target?
Depth-first or breadth-first traversal — does the order matter for a real use case?
Return a live-updating collection (like the real native method does) or a static snapshot array?

Code / implementation expected: Yes — real, direct proof against the REAL native getElementsByClassName, confirming identical results on a real nested DOM tree.

js-coding
utility
algorithm

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Solution

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

How to read this doc: correctness was verified the strongest possible way — by comparing the custom implementation output DIRECTLY against the REAL, native getElementsByClassName on the identical real DOM tree, not just checking it against a hand-written expected list.

1. The problem, restated

Recursively walk a DOM subtree starting from a given root, collecting every descendant element whose class attribute includes the target class name — matching the real, native getElementsByClassName's own documented behavior (including elements with MULTIPLE classes, one of which matches).

2. Clarifying questions a strong candidate asks

Question	Why it matters
Multiple classes on one element?	Yes, genuinely required — `classList.contains` (or a

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: the custom recursive implementation matches the real, native getElementsByClassName exactly, on a real nested DOM tree

Practice

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