Premium

Deep Equal

By FrontendAtlas Team · Updated Jan 30, 2026

Implement a function deepEqual(a, b) that returns true when two values are deeply equal — primitives by value and objects/arrays by recursively comparing their properties. Handle Dates, RegExps, and NaN.

What you’ll build / What this tests

This premium javascript coding focuses on Deep Equal. You’ll apply comparison and equality thinking with hard level constraints. The prompt emphasizes Implement a function deepEqual(a, b) that returns true when two values are deeply equal — primitives….

Learning goals

Translate the prompt into a clear javascript API signature and return shape.
Apply comparison, equality, objects techniques to implement deep equal.
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.
Avoid prototype pitfalls when reading object keys.
Avoid mutating nested objects; return new references.
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 a = /* deep equal input */;
const b = /* config */;
const result = deepEqual(a, b);
console.log(result);

// Edge case check
const empty = a && b ?? null;
const fallback = deepEqual(a, b);
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.