useEffectOnce: Run an Effect Only Once

What you’ll build / What this tests

This premium react coding focuses on useEffectOnce: Run an Effect Only Once. You’ll apply react and hooks thinking with medium level constraints. The prompt emphasizes This hook should run the effect only on mount and clean up on unmount, even under….

Learning goals

• Translate the prompt into a clear react API signature and return shape.
• Apply react, hooks, effects techniques to implement useeffectonce: run an effect only once.
• Handle medium edge cases without sacrificing readability.
• Reason about time/space complexity and trade-offs in react.

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

• Implement useEffectOnce(effect) in useEffectOnce.ts.
• The effect callback must run only once for the lifetime of a mounted component.
• If effect returns a cleanup function, it must run exactly once on unmount.
• The hook should be safe under React 18 StrictMode (dev): no duplicate real side-effects after the initial mount.
• Mounting the demo triggers the effect exactly once (count increments from 0 to 1).
• Re-rendering the component does NOT re-run the effect.
• Unmounting runs cleanup exactly once (count increments by 100 in the demo).
• Use useEffect internally.
• Use useRef to persist flags/cleanup between renders.
• Avoid useMemo for running effects. Effects belong in useEffect.

Mini snippet (usage only)

// Example usage
const input = /* useeffectonce: run an effect only once input */;
const result = solve(input);
console.log(result);

// Edge case check
const empty = input ?? null;
const fallback = solve(input);
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.

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