🔥 Introduction
React 19 is finally here — and it’s the biggest update since Hooks. With the new Actions API, improved
handling, Asset Loading APIs, and the React Compiler, it’s changing the way we build apps.If you’re preparing for frontend interviews in 2025, this post is your ultimate React 19 prep kit. We’ll break down what’s new, migration tips, code demos, and 20 interview questions with answers.
⚡ What’s New in React 19
- Actions API (Server Actions)
React 19 introduces Actions, making server interactions first-class.
'use client'
import { useActionState } from 'react'
async function createTodoAction(formData) {
"use server"
const title = formData.get("title")
// save to DB
}
export default function TodoForm() {
const [state, formAction] = useActionState(createTodoAction)
return (
Add
)
}
✅ No more manual fetch → form → handler boilerplate.
- Improved Handling
Forms now work without client-side JS. Progressive enhancement by default!
- Asset Loading APIs
Preload images, fonts, and scripts easily:
- React Compiler (Preview)
Transforms your JSX into highly optimized code → reduces re-renders automatically.
- Cleanup of Legacy APIs
Old stuff like ReactDOM.render() → gone. Use createRoot() instead.
⚖️ React 19 vs React 18
Feature React 18 React 19
Server Actions ❌ ✅
Form submission Manual handlers Built-in
Asset Loading Manual preload API-based
Compiler ❌ Preview
Legacy APIs Present Removed
🧑💻 Migration Tips
Replace ReactDOM.render() → createRoot()
Audit forms → adopt new
behaviorTest asset preloading → integrate preload/preconnect
Start experimenting with Compiler, but keep fallback
🎯 20 React 19 Interview Questions
What is the Actions API in React 19?
→ A first-class way to handle server mutations directly in React components.
How does useActionState work?
→ Hook to track state of a server action (loading, success, error).
What’s the benefit of React 19
changes?→ Forms submit without client JS, making apps faster & more resilient.
How does the new Compiler optimize code?
→ It transforms JSX to reduce unnecessary re-renders automatically.
What is removed in React 19?
→ Legacy APIs like ReactDOM.render().
Compare React 18 vs React 19 in terms of server actions.
→ React 18 had no built-in support, React 19 introduces Actions API.
Can React 19 forms work without JavaScript enabled?
→ Yes, progressive enhancement allows it.
What is “use server” in React 19?
→ Directive marking functions as server-only.
Explain asset loading in React 19.
→ Built-in APIs for preloading fonts, images, and scripts.
Why is the React Compiler important?
→ Automates performance optimization without developer intervention.
How do server actions improve DX?
→ Removes need for separate API route handling.
Give an example of a breaking change in React 19.
→ Removal of ReactDOM.render().
Is React 19 backward compatible with React 18?
→ Mostly yes, but with some breaking changes (e.g., legacy APIs).
What’s the difference between server actions and client actions?
→ Server actions run on the server; client actions run in browser JS.
How does form error handling work in React 19?
→ Errors can be returned directly from server actions.
Why is progressive enhancement important?
→ Ensures apps work even with JS disabled.
Can you use Compiler + useMemo/useCallback together?
→ Compiler aims to reduce the need, but you still can.
What changes in Suspense in React 19?
→ Better asset loading integration.
What happens if you don’t migrate legacy APIs?
→ Your app may break after upgrading.
How do you upgrade to React 19 safely?
→ Test forms, replace legacy APIs, enable compiler gradually.
✅ Conclusion
React 19 isn’t just an upgrade—it’s a paradigm shift. With server actions, smarter forms, built-in asset loading, and an optimizing compiler, React 19 redefines frontend dev.
👉 If you’re preparing for interviews in 2025, master these features now.
📌 For a complete roadmap: FullStackPrep.dev
📌 Suggested Tags
React19, ReactJS, WebDevelopment, Frontend, InterviewPrep, JavaScript, FullStack, CodingInterviews