I spent over a year writing backend code in Flask. When I started a new project, I decided to try Next.js. The first thing I noticed was déjà vu. This felt like Flask, rewritten in TypeScript with a newer take on rendering.
React itself works well for me - a tidy component library, nothing more. Next.js is what I fell for, the App Router most of all. The deeper I dig, the more it looks like a classic server-side framework, with extras Flask would never build on its own.
My Flask experience isn't from tutorials. I built FileServer on it (the project is moving to FastAPI now, since the old code stopped scaling). That work is what showed me how much these two tools overlap in concept.
Templating and layouts
In Flask with Jinja, you build a base.html with a {% block content %} block, and users.html extends that template. Header, footer, navigation - written once, not copied on every page.
Next.js does the same job with layout.tsx. You set it at the folder level, and every page inside wraps in it, no extra code needed. App Router lets you nest layouts at any depth - /admin gets its own layout on top of the shared one. Flask can do this too, through several levels of {% extends %}, but by hand it feels less clean.
Server rendering by default
My first guess: TSX is Jinja with a different syntax. Close, but not exact.
A component in Next.js defaults to a Server Component. It renders on the server, and the browser gets finished HTML, the same way it does after Jinja.
A component turns client-side only when you add 'use client' at the top of the file. That's the real parallel with Flask. Flask never gives you the choice - everything runs on the server. Next.js gives you the choice, and you can mix server markup with interactive pieces in the browser.
Data fetching
In Flask, you write the data-fetching logic right in the route, then pass it to the template:
@app.route('/users')
def users():
users = db.get_all_users()
return render_template('users.html', users=users)
In Next.js, Server Components let you do the same thing inside the component itself, with no extra API endpoint to build:
// app/users/page.tsx
export default async function UsersPage() {
const users = await db.user.findMany();
return <ul>{users.map(u => <li key={u.id}>{u.name}</li>)}</ul>;
}
It still feels like writing backend code.
Dynamic routes
Flask: @app.route('/user/<int:user_id>'), and user_id arrives in the function as an argument.
Next.js uses the same idea with square brackets in the folder name: app/user/[id]/page.tsx, and you pull it out as params.id. Different syntax, same idea - a piece of the URL becomes a variable.
Form handling
In Flask, you build a form, set method="POST", and catch the data on the backend through request.form.
Next.js added Server Actions. You write an async function on the server and attach it to the <form> tag with no extra wiring:
async function createUser(formData: FormData) {
const name = formData.get('name');
await db.user.create({ data: { name } });
}
<form action={createUser}>
<input name="name" />
<button type="submit">Create</button>
</form>
It's the same old POST request, packaged in newer syntax. You don't need a separate API route for simple mutations.
API and HTML in one app
A single Flask app holds /users, which renders HTML, next to /api/users, which runs jsonify(...) and returns plain JSON. One server, two kinds of response.
Next.js works the same way: app/users/page.tsx returns a JSX page, and the neighboring app/api/users/route.ts returns JSON. This is also how I plan to build the FastAPI backend for FileServer - API on one side, frontend on the other, an idea I picked up from the Flask version of the project.
Request lifecycle: middleware
Who sees a request before it reaches your code? In Flask, @app.before_request handles that - a function that runs before every route. Useful for checking auth or logging.
Next.js has middleware.ts at the project root for the same job. It catches the request before it reaches the page, and that's where you handle redirects or check a cookie. It matches before_request almost exactly.
Grouping routes
Next.js has Route Groups - a folder in parentheses like (auth) that groups routes logically without showing up in the URL.
Flask solves the same problem with Blueprints: move the auth routes into one blueprint, the admin routes into another, then register both on the main app. The mechanics differ, but the motivation matches - don't dump every route into one flat pile.
Caching and static files
In Flask, to keep from hitting the database on every request, you add a @cache.cached(timeout=50) decorator.
Next.js builds this into the core, under the name ISR (Incremental Static Regeneration). You export a config variable:
export const revalidate = 60;
Redirects and request context
In Flask, you use redirect(url_for('login')) to send a user elsewhere. For headers, you import the global request object.
In Next.js server code, you do the same thing through dedicated functions instead:
import { redirect } from 'next/navigation';
import { headers } from 'next/headers';
if (!session) redirect('/login');
const token = headers().get('Authorization');
The takeaway
People often file Next.js under "yet another SPA framework." Under the hood, App Router runs a solid server framework that does what Flask does, then lets you drop in client-side JavaScript exactly where you need it. If you know Flask well, Next.js won't take long to pick up. The concepts stay the same. Only the syntax changed.
