Summary
If you're migrating tests to React 19, this gives you the exact before/after patterns for the breaking changes. It covers the act() import move from react-dom/test-utils to react, replacing the removed Simulate API with fireEvent, and the StrictMode call count adjustments since effects no longer double-invoke in development. The priority order is smart: fix act() imports first since everything else depends on it, then work through Simulate, then tackle the call count assertions. The StrictMode section is honest about measuring actual counts instead of guessing, which will save you from chasing phantom bugs when your spies expect 2 calls but now get 1.
Install to Claude Code
npx -y skills add github/awesome-copilot --skill react19-test-patterns --agent claude-codeInstalls into .claude/skills of the current project.
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belt cli automatically finds the best tools and skills for your agent. image, video, music, tts...
one prompt install →Plug Mailtrap into your AI workflow and let it handle the email.
Connect Mailtrap MCP →Agent, run crypto. Access onchain data & trade routes via 1inch.
Install now →Make coding agent sessions - Searchable, Shareable, Vendor-neutral & Scored.
Try For Free →Your agent targets a perfect 10 Code Health score. Deterministic. Every commit.
Try For Free →Integrate web data into your AI product. One API to scrape website & brand data.
Get API Key Now →First SeenMay 16, 2026
