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Group By

By FrontendAtlas Team · Updated Jan 31, 2026

Group an array of items by a key (function or property). Build an object or Map where each key maps to an array of items, and preserve input order inside each group. Concepts: arrays, grouping, objects.

What you’ll build / What this tests

This premium javascript coding focuses on Group By. You’ll apply arrays and grouping thinking with intermediate level constraints. The prompt emphasizes Group an array of items by a key (function or property). Build an object or Map….

Learning goals

Translate the prompt into a clear javascript API signature and return shape.
Apply arrays, grouping, objects techniques to implement group by.
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.
Avoid prototype pitfalls when reading object keys.
Do not mutate input arrays; preserve item order.
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 arr = /* group by input */;
const keyFn = /* config */;
const result = groupBy(arr, keyFn);
console.log(result);

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