Router level middleware in Express - Step-by-Step Execution

Concept Flow - Router-level middleware

Request comes in

↓

Router receives request

↓

Router-level middleware runs

↓

Middleware calls next()

↓

Router handles route or passes to next middleware

↓

Response sent or next middleware runs

When a request hits a router, router-level middleware runs first. It can modify the request or response, then calls next() to continue to route handlers or other middleware.

Execution Sample

Express

const express = require('express');
const router = express.Router();

router.use((req, res, next) => {
  console.log('Middleware runs');
  next();
});

router.get('/', (req, res) => res.send('Hello'));

module.exports = router;

This code sets up router-level middleware that logs a message before handling a GET request.

Execution Table

Step	Action	Middleware Called	next() Called	Route Handler Called	Output
1	Request to '/' received by router	No	No	No	No output yet
2	Router-level middleware runs	Yes	Yes	No	Logs 'Middleware runs'
3	Route handler for '/' runs	No	No	Yes	Sends 'Hello' response
4	Response sent to client	No	No	No	Client receives 'Hello'

💡 Response sent after route handler completes, middleware chain ends

Variable Tracker

Variable	Start	After Middleware	After Route Handler	Final
req.url	/	/	/	/
res.headersSent	false	false	true	true
middlewareCalled	false	true	true	true
responseSent	false	false	true	true

Key Moments - 2 Insights

Why does the route handler run only after middleware calls next()?

Because in the execution_table row 2, middleware calls next(), which tells Express to continue to the next function, the route handler.

What happens if middleware does not call next()?

The request stops at middleware and the route handler never runs, so no response is sent (see execution_table step 2 importance of next()).

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the middleware log 'Middleware runs'?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Check the 'Middleware Called' and 'Output' columns in execution_table row 2

According to variable_tracker, when does res.headersSent become true?

A After middleware runs

B After route handler runs

C At the start

D Never

💡 Hint

Look at 'res.headersSent' values in variable_tracker after route handler

If middleware did not call next(), what would happen to the route handler?

A It would run anyway

B It would run before middleware

C It would not run

D It would run twice

💡 Hint

Refer to key_moments explanation about next() importance

Concept Snapshot

Router-level middleware runs when a request hits a router.
It can modify req or res, then must call next() to continue.
If next() is not called, request stops and route handlers don't run.
Use router.use() to add middleware to a router.
Middleware runs before route handlers inside that router.

Full Transcript

Router-level middleware in Express runs when a request is received by a router. The middleware function can do tasks like logging or modifying the request. It must call next() to pass control to the next middleware or route handler. If next() is not called, the request stops and no response is sent. After middleware calls next(), the router's route handler runs and sends the response. Variables like req.url stay the same, while res.headersSent changes to true after the response is sent. This flow ensures middleware can prepare or check requests before routes handle them.