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.
Keeps your Mac awake while Claude Code, Codex or Cursor works. Lets it sleep when they are done.
Try free for 7 days →
Connect your Claude agent to live crypto prices and trading routes via 1inch
Get the MCP →
Block distracting apps from your iPhone permanently without a 3rd party app. Free and open source.
Block now (100% free) →
belt cli automatically finds the best tools and skills for your agent. image, video, music, tts...
one prompt install →
Free an open-source skills that make AI coding agents easier to understand, verify, and control.
Download for free →Files
SKILL.md
Select a file.
Featured
Keeps your Mac awake while Claude Code, Codex or Cursor works. Lets it sleep when they are done.
Try free for 7 days →Connect your Claude agent to live crypto prices and trading routes via 1inch
Get the MCP →Block distracting apps from your iPhone permanently without a 3rd party app. Free and open source.
Block now (100% free) →belt cli automatically finds the best tools and skills for your agent. image, video, music, tts...
one prompt install →Free an open-source skills that make AI coding agents easier to understand, verify, and control.
Download for free →First SeenMay 16, 2026
