React 17 to React 18 Upgrade

When to Use This Skill

• User wants to upgrade existing React 17 app to 18
• Console warnings about ReactDOM.render is no longer supported
• Preparing codebase for React 19
• Fixing issues after partial upgrade to concurrent features

Upgrade Phases

Follow one phase at a time. Validate after each (build, test, dev server, console). Use grep/read before edits.

See references/ for detailed guidance, code examples, and troubleshooting.