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Implement Promise.all

By FrontendAtlas Team · Updated Jan 30, 2026

Implement a function promiseAll(promises) that behaves like Promise.all. It should take an array of values or promises and return a single promise that resolves to an array of results, preserving order, or rejects as soon as one promise rejects.

What you’ll build / What this tests

This premium javascript coding focuses on Implement Promise.all. You’ll apply promise and async thinking with intermediate level constraints. The prompt emphasizes Implement a function promiseAll(promises) that behaves like Promise.all. It should take an array of values or….

Learning goals

Translate the prompt into a clear javascript API signature and return shape.
Apply promise, async, concurrency techniques to implement implement promise.all.
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.
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

Preserve input order and handle empty arrays safely.
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 promises = /* implement promise.all input */;
const result = promiseAll(promises);
console.log(result);

// Edge case check
const empty = promises ?? null;
const fallback = promiseAll(promises);
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.