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Compose Function

By FrontendAtlas Team · Updated Jan 30, 2026

Implement compose(...fns) that returns a new function applying the provided functions from right to left. The rightmost function may accept multiple arguments; every other function is unary and receives the previous result. If no functions are provided, return an identity function (x) => x.

What you’ll build / What this tests

This premium javascript coding focuses on Compose Function. You’ll apply composition and functions thinking with intermediate level constraints. The prompt emphasizes Implement compose(...fns) that returns a new function applying the provided functions from right to left. The….

Learning goals

Translate the prompt into a clear javascript API signature and return shape.
Apply composition, functions, higher-order techniques to implement compose function.
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.
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

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.
Be explicit about edge cases and error states.
Prioritize clarity over cleverness in explanations.
Explain the trade-offs behind your choices.

Mini snippet (usage only)

// Example usage
const fns = /* compose function input */;
const result = compose(fns);
console.log(result);

// Edge case check
const empty = fns ?? null;
const fallback = compose(fns);
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.