What problem does it solve? React Strict Mode double-invokes setState updater functions in development, which exposes hidden bugs where updaters read refs, mutate external variables, or trigger side effects. These bugs cause state inconsistencies that only appear in development or during rapid consecutive interactions, making them hard to reproduce and fix. ## Core Features & Use Cases - Detection Rules: Concrete patterns for spotting impure updaters, including ref reads/writes, external mutable state, non-deterministic values like Date.now or Math.random, and side effects such as logging or API calls inside updaters. - Fix Templates: Three ready-to-apply refactoring templates covering ref capture outside the updater, pre-computing non-deterministic values, and moving side effects into event handlers. - Regression Testing: A minimal Vitest and React Testing Library test pattern that wraps components in StrictMode and verifies consecutive operations produce independent, correct state. - Use Case: A button that adds candidates to a form works once but overwrites state on the second click in development. Use this Skill to write a failing test, extract the ref access out of the updater, and lock in a two-click regression test. ## Quick Start Ask the AI to review my React component for impure setState updater functions and fix them using the strict mode updater safety workflow with a failing test first.
