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Debug: Component Renders Twice on Every Update

By FrontendAtlas Team · Updated Jan 31, 2026

Fix a component that renders twice per click because it stores derived state in useEffect. The correct fix is to remove the derived parity state and compute it directly from count during render, preventing the extra state update and render. Explain StrictMode double-invocation, idempotent effects,…

Work in src/App.tsx.
Keep the UI structure and existing styles.

What you’ll build / What this tests

This premium react coding focuses on Debug: Component Renders Twice on Every Update. You’ll apply react and debugging thinking with intermediate level constraints. The prompt emphasizes Fix a component that renders twice per click because it stores derived state in useEffect. The….

Learning goals

Translate the prompt into a clear react API signature and return shape.
Apply react, debugging, rendering techniques to implement debug: component renders twice on every update.
Handle intermediate 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

Work in src/App.tsx.
Keep the UI structure and existing styles.
Clicking "Increment" should increase the count by 1.
The parity label must remain correct (even/odd).
A single click should result in only one additional render (render count increases by 1).
Count increases by 1 when Increment is clicked.
Parity updates correctly to "even" or "odd".
Render count increases by 1 per click (not 2).
Reset returns the counter to 0 and parity to even.
Avoid syncing derived state with useEffect.

Mini snippet (usage only)

// Example usage
const input = /* debug: component renders twice on every update 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.