Protecting routes with auth middleware in Express - Step-by-Step Execution

Concept Flow - Protecting routes with auth middleware

Incoming Request

↓

Auth Middleware

↓

Auth OK

↓

Next Route

↓

Response

The request first passes through auth middleware which checks credentials. If valid, it proceeds to the route handler; if not, it sends a 401 Unauthorized response.

Execution Sample

Express

function auth(req, res, next) {
  if (req.user) next();
  else res.status(401).send('Unauthorized');
}

app.get('/dashboard', auth, (req, res) => {
  res.send('Welcome!');
});

This code checks if a user is logged in before allowing access to the /dashboard route.

Execution Table

Step	Request URL	req.user	Middleware Action	Route Handler Called	Response Sent
1	/dashboard	undefined	Check req.user -> false	No	401 Unauthorized
2	/dashboard	{id:1}	Check req.user -> true	Yes	Welcome!

💡 Execution stops after sending response either from middleware (401) or route handler (Welcome!)

Variable Tracker

Variable	Start	After Step 1	After Step 2
req.user	undefined	undefined	{id:1}
next called	false	false	true
response status	none	401	200
response body	none	'Unauthorized'	'Welcome!'

Key Moments - 2 Insights

Why does the route handler not run when req.user is undefined?

Because the middleware sends a 401 response and does not call next(), stopping further execution as shown in execution_table step 1.

What does calling next() inside middleware do?

Calling next() passes control to the next middleware or route handler, allowing the request to continue as shown in execution_table step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what response is sent when req.user is missing?

A 401 Unauthorized

B Welcome!

C 404 Not Found

D 500 Internal Server Error

💡 Hint

Check execution_table row 1 under 'Response Sent'

At which step does the route handler get called?

A Step 1

B Step 2

C Neither step

D Both steps

💡 Hint

Look at 'Route Handler Called' column in execution_table

If middleware always calls next() without checking req.user, what happens?

A Middleware blocks all requests

B Server crashes

C Unauthorized users access the route

D Response is always 401

💡 Hint

Think about what happens if middleware never sends 401 and always calls next()

Concept Snapshot

Protect routes by adding auth middleware before route handlers.
Middleware checks if user is logged in (req.user).
If yes, call next() to continue.
If no, send 401 Unauthorized response.
This stops unauthorized access simply and clearly.

Full Transcript

In Express, protecting routes means checking if a user is logged in before allowing access. We write a middleware function that looks at req.user. If req.user exists, it calls next() to let the request continue to the route handler. If not, it sends a 401 Unauthorized response and stops. This way, only authorized users reach the route. The execution table shows two cases: one where req.user is missing and the middleware sends 401, and one where req.user exists and the route handler sends a welcome message. Key points are that calling next() passes control forward, and not calling next() stops the request. If middleware always calls next() without checking, unauthorized users can access protected routes. This pattern keeps routes safe with simple checks.