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

Concept Flow - Application-level middleware

Request Received

↓

Middleware 1 Executes

↓

Middleware 2 Executes

↓

Route Handler Executes

↓

Response Sent

↓

End

The request passes through each middleware in order, then reaches the route handler, which sends the response.

Execution Sample

Express

const express = require('express');
const app = express();

app.use((req, res, next) => {
  console.log('Middleware 1');
  next();
});

app.get('/', (req, res) => res.send('Hello World!'));

This code sets up an Express app with one middleware that logs a message, then a route that sends a response.

Execution Table

Step	Action	Middleware/Handler	Console Output	Next Called	Response Sent
1	Request received	-	-	-	-
2	Middleware 1 runs	Middleware 1	Middleware 1	Yes	-
3	Route handler runs	Route Handler for '/'	-	-	Hello World!
4	Response sent	-	-	-	Hello World!
5	Request cycle ends	-	-	-	-

💡 Response sent by route handler, request cycle ends.

Variable Tracker

Variable	Start	After Middleware 1	After Route Handler	Final
req	Incoming request object	Passed through Middleware 1	Handled by Route Handler	Request cycle complete
res	Response object	Passed through Middleware 1	Response sent	Response cycle complete
next	Function to call next middleware	Called to continue	Not called (end of chain)	No further middleware

Key Moments - 2 Insights

Why does the route handler run after the middleware?

Because the middleware calls next(), which tells Express to continue to the next middleware or route handler, as shown in step 2 of the execution_table.

What happens if middleware does not call next()?

The request stops there and the route handler never runs, so no response is sent. This is why next() is important, as seen in the execution_table where next is called at step 2.

Visual Quiz - 3 Questions

Look at the execution table, what is logged to the console at step 2?

A "Middleware 1"

B "Route Handler"

C "Hello World!"

D Nothing

💡 Hint

Check the 'Console Output' column at step 2 in the execution_table.

At which step is the response sent to the client?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Response Sent' column in the execution_table.

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

A It would run immediately

B It would run twice

C It would never run

D It would send two responses

💡 Hint

Refer to the key_moments section about the importance of calling next().

Concept Snapshot

Application-level middleware in Express runs in order for each request.
Each middleware receives req, res, and next.
Middleware must call next() to pass control.
Route handlers run after middleware chain.
Response is sent by route handler or middleware.
Middleware can modify req/res or end response early.

Full Transcript

Application-level middleware in Express is a function that runs for every request to the app or specific routes. When a request comes in, Express runs middleware functions in the order they were added. Each middleware receives the request and response objects, plus a next function. Calling next() passes control to the next middleware or route handler. If next() is not called, the request stops and no further processing happens. After middleware, the route handler runs and sends the response. This flow ensures middleware can modify requests or responses before the final handler sends data back to the client.