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Flatten with Depth

By FrontendAtlas Team · Updated Jan 31, 2026

Implement flatten(arr, depth) that flattens nested arrays up to a specific depth. Depth controls how many nesting levels to remove: depth = 0 returns a shallow copy; depth = 1 flattens one level. Handle mixed primitives and arrays.

What you’ll build / What this tests

This premium javascript coding focuses on Flatten with Depth. You’ll apply arrays and depth thinking with intermediate level constraints. The prompt emphasizes Implement flatten(arr, depth) that flattens nested arrays up to a specific depth. Depth controls how many….

Learning goals

Translate the prompt into a clear javascript API signature and return shape.
Apply arrays, depth, flatten techniques to implement flatten with depth.
Handle intermediate 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.
Do not mutate input arrays; preserve item order.
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 arr = /* flatten with depth input */;
const depth = /* config */;
const result = flatten(arr, depth);
console.log(result);

// Edge case check
const empty = arr && depth ?? null;
const fallback = flatten(arr, depth);
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.