Premium

DOM Tree Walk 3: Basic Selector Engine (Hard)

By FrontendAtlas Team · Updated Jan 31, 2026

Implement a small selector engine: queryAll(root, selector) that returns all matching nodes in document order from a DOM-like tree. This is a classic "build a tiny querySelectorAll" interview task. DOM-like node shape: - nodeType: 1 for element, 3 for text - tagName: string for elements…

What you’ll build / What this tests

This premium javascript coding focuses on DOM Tree Walk 3: Basic Selector Engine (Hard). You’ll apply dom and tree thinking with hard level constraints. The prompt emphasizes Implement a small selector engine: queryAll(root, selector) that returns all matching nodes in document order from….

Learning goals

Translate the prompt into a clear javascript API signature and return shape.
Apply dom, tree, selectors techniques to implement dom tree walk 3: basic selector engine (hard).
Handle hard edge cases without sacrificing readability.
Reason about time/space complexity and trade-offs in javascript.

Key decisions to discuss

Define the exact input/output contract before coding.
Choose iteration vs higher-order methods for readability.
Prioritize predictable edge-case handling over micro-optimizations.

Evaluation rubric

Correctness: covers required behaviors and edge cases.
Clarity: readable structure and predictable control flow.
Complexity: avoids unnecessary work for large inputs.
API discipline: no mutation of inputs; returns expected shape.
Testability: solution is easy to unit test.

Constraints / Requirements

Preserve input order and handle empty arrays safely.
Handle empty strings and mixed casing without errors.
Avoid deep recursion issues on large inputs.
Do not access the real DOM; use the provided node shape.
Handle empty or missing inputs without throwing errors.
Keep runtime close to linear time where possible.
Prefer a pure function: no side effects beyond the return value.

Mini snippet (usage only)

// Example usage
const root = /* dom tree walk 3: basic selector engine (hard) input */;
const selector = /* config */;
const result = queryAll(root, selector);
console.log(result);

// Edge case check
const empty = root && selector ?? null;
const fallback = queryAll(root, selector);
console.log(fallback);

// Expected: describe output shape, not the implementation
// (no solution code in preview)

Common pitfalls

Mutating inputs instead of returning a new value.
Skipping edge cases like empty input, duplicates, or nulls.
Overlooking time complexity for large inputs.