Creating middleware in NestJS - Why You Should Know This

The Big Idea

What if you could fix a bug in one place and instantly improve your whole app's security?

The Scenario

Imagine you have to check user login status on every single route in your app by writing the same code again and again inside each route handler.

The Problem

Manually repeating checks everywhere makes your code messy, hard to update, and easy to forget important steps, leading to bugs and security holes.

The Solution

Middleware lets you write that check once and apply it automatically to many routes, keeping your code clean and consistent.

Before vs After

✗ Before

@Get('/profile')
getProfile(@Req() req, @Res() res) {
  if (!req.user) return res.status(401).send('Login required');
  /* handler code */
}
@Get('/settings')
getSettings(@Req() req, @Res() res) {
  if (!req.user) return res.status(401).send('Login required');
  /* handler code */
}

✓ After

app.use(authMiddleware);
@Get('/profile')
profileHandler() {
  /* handler code */
}
@Get('/settings')
settingsHandler() {
  /* handler code */
}

What It Enables

You can easily add, remove, or change shared logic for many routes in one place, making your app safer and easier to maintain.

Real Life Example

In a shopping app, middleware can check if a user is logged in before letting them add items to their cart or checkout.

Key Takeaways

Writing repeated code for shared tasks is slow and risky.

Middleware centralizes common logic for many routes.

This keeps your app clean, secure, and easy to update.