What are the most important changes in React 19?

Summary

React 19 introduced Actions (async transitions for mutations) with useActionState, useFormStatus and useOptimistic; the use() API for reading promises and context; ref as a regular prop (no forwardRef needed); native document metadata (<title>, <meta>) and stylesheet/script support; resource preloading APIs; and stable Server Components and Server Actions.

Longer Answer

It also improved hydration error messages, allowed <Context> as a provider, supports cleanup functions returned from ref callbacks, and removed legacy APIs (string refs, propTypes checks, defaultProps on function components, legacy context). React 19.2 added <Activity> and useEffectEvent. The React Compiler (separately released) automates memoization.

Key Points

• Actions + useActionState/useFormStatus/useOptimistic
• use() for promises and context
• ref as a prop
• Document metadata and resource preloading
• Removed legacy APIs

What interviewers look for

Concrete knowledge of the new form/mutation story and what changed for existing code.

Follow-up questions

• How would you migrate a form from useState + fetch to Actions?