Premium

Concurrency-Limited Map (order-preserving)

By FrontendAtlas Team · Updated Jan 31, 2026

Implement a concurrency-limited async map that preserves order. Run at most N promises at a time, resolve results in input order, and reject early on errors to avoid hanging work.

What you’ll build / What this tests

This premium javascript coding focuses on Concurrency-Limited Map (order-preserving). You’ll apply async and concurrency thinking with hard level constraints. The prompt emphasizes Implement a concurrency-limited async map that preserves order. Run at most N promises at a time,….

Learning goals

Translate the prompt into a clear javascript API signature and return shape.
Apply async, concurrency, promise techniques to implement concurrency-limited map (order-preserving).
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.
Decide on concurrency and error propagation behavior.
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

Avoid prototype pitfalls when reading object keys.
Handle async flow without blocking the event loop.
Return a Promise and resolve asynchronously without blocking.
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 items = /* concurrency-limited map (order-preserving) input */;
const limit = /* config */;
const result = await mapAsyncLimit(items, limit);
console.log(result);

// Edge case check
const empty = items && limit ?? null;
const fallback = await mapAsyncLimit(items, limit);
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.
Forgetting to await or return the Promise.
Overlooking time complexity for large inputs.