Everything I Wish I Knew Before Migrating My First Vite Project to Next.js

The Shift from Client-Side to Server-Side

When I first started building with React, Vite was my go-to. Its lightning-fast HMR (Hot Module Replacement) and simple configuration made it the gold standard for Single Page Applications (SPAs). However, as my project grew, I hit the inevitable walls of SEO limitations and large bundle sizes that slowed down initial page loads. The decision to migrate to Next.js was clear, but the execution was far more complex than I anticipated.

Migrating isn't just about changing a configuration file; it’s a fundamental shift in how your application thinks about data, routing, and the browser. Here is everything I wish I had known before I typed npm install next.

1. The "Window is not defined" Nightmare

In a Vite-powered SPA, you can safely assume the window object is always available because the code only runs in the browser. In Next.js, your components are pre-rendered on the server by default.

I spent hours debugging third-party libraries that tried to access localStorage or window at the top level. To fix this, you must wrap browser-only logic in a useEffect hook or use dynamic imports with ssr: false.

// Instead of this:
const token = localStorage.getItem('auth');

// Do this:
useEffect(() => {
  const token = localStorage.getItem('auth');
}, []);

2. File-System Routing vs. React Router

If you are coming from Vite, you likely have a Routes.tsx file with dozens of <Route /> components. Next.js uses the file system to define routes.

• pages/index.tsx becomes your / route.
• pages/about.tsx becomes /about.

If you are using the newer App Router, this changes to a directory-based structure (app/about/page.tsx). Manually mapping 50+ routes from a complex react-router-dom setup is one of the most tedious parts of the transition. If you're looking to automate this tedious plumbing, you might find a tool like ViteToNext.AI useful for handling the structural conversion of components and routes automatically.

3. The Image Component is Non-Negotiable

I initially thought I could just keep using standard <img> tags. While they work, you lose one of Next.js's greatest benefits: automatic image optimization.

Switching to next/image requires you to define widths and heights (or use fill) to prevent Layout Shift (CLS). It forces a discipline on you that Vite doesn't, but the performance gains in Core Web Vitals are massive.

4. API Routes and Data Fetching

In Vite, I used axios inside useEffect to fetch data from an external backend. In Next.js, you have choices: getStaticProps, getServerSideProps, or (in the App Router) Server Components.

I wish I knew that I didn't have to move my entire backend into Next.js API routes immediately. You can still fetch from your legacy API, but doing so on the server side means your API keys stay hidden and the user gets a fully rendered page without a loading spinner.

5. CSS and Styling

If you are using CSS Modules, the transition is seamless. However, if you are using global CSS imports inside specific components, Next.js will complain. Global CSS can only be imported in _app.tsx (Pages Router) or layout.tsx (App Router).

If your Vite project relied heavily on styled-components or Emotion, you’ll need to set up a specific registry to ensure styles are injected correctly during SSR, otherwise, you'll see a flash of unstyled content (FOUC).

6. Environment Variables

In Vite, environment variables are prefixed with VITE_. In Next.js, they must be prefixed with NEXT_PUBLIC_ if you want them to be accessible in the browser. I spent a solid thirty minutes wondering why my Firebase config was undefined before realizing I hadn't renamed my keys.

Conclusion

Migrating from Vite to Next.js is like moving from a nimble sports car to a robust SUV. You lose a bit of that "raw" simplicity, but you gain the power to handle much larger, more complex loads with better performance and SEO.

Take it slow, migrate one route at a time, and embrace the Server Component mental model early. It will save you significant refactoring time in the long run.

Further reading: How to automate your Vite to Next.js migration