Morgan for HTTP request logging in Express - Step-by-Step Execution

Concept Flow - Morgan for HTTP request logging

Incoming HTTP Request

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Morgan Middleware Intercepts

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Start Timer & Pass to Next

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Route Handler Responds

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Format & Output Log on 'finish'

Morgan middleware catches each HTTP request, starts a timer, passes control to the next middleware/route handler. When the response 'finish' event fires, it formats a log line and outputs it.

Execution Sample

Express

const express = require('express');
const morgan = require('morgan');
const app = express();
app.use(morgan('tiny'));
app.get('/', (req, res) => res.send('Hello'));
app.listen(3000);

This code sets up an Express server that logs each HTTP request in a short format using Morgan.

Execution Table

Step	Action	Request Method	Request URL	Morgan Log Output	Next Step
1	HTTP GET request arrives	GET	/	—	Morgan middleware intercepts
2	Morgan starts timer	GET	/	—	Pass control to route handler
3	Route handler sends response	GET	/	—	Response 'finish' event
4	Morgan formats log entry	GET	/	GET / 200 - - 3.45 ms	Output log to console
5	Request complete	GET	/	—	End of request cycle

💡 Request completes after response sent; Morgan logs each request on response 'finish' event.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
req.method	undefined	GET	GET	GET	GET
req.url	undefined	/	/	/	/
morganLog	undefined	undefined	undefined	undefined	GET / 200 - - 3.45 ms

Key Moments - 2 Insights

Why does Morgan log after the route handler sends the response?

Morgan logs on the response 'finish' event (Step 4 in execution_table), after the route handler (Step 3). This captures status code and response time accurately.

What does the 'tiny' format in Morgan show in the log?

The 'tiny' format outputs method, URL, status code, response time, and a few dashes for missing info. See Morgan Log Output column in Step 4 for example.

Visual Quiz - 3 Questions

Look at the execution table, what is the Morgan log output at Step 4?

A GET / 200 - - 3.45 ms

B POST /api 404 - - 10 ms

C Error: Request failed

D No log output yet

💡 Hint

Check the 'Morgan Log Output' column at Step 4 in the execution_table.

At which step does Morgan pass control to the next middleware or route handler?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Next Step' column to see when control passes to route handler.

If the request method was POST instead of GET, how would the Morgan log output change at Step 4?

A It would still show GET / 200 - - 3.45 ms

B Morgan would not log POST requests

C It would show POST / 200 - - 3.45 ms

D It would show GET / 404 - - 3.45 ms

💡 Hint

Morgan logs the actual request method from req.method as shown in variable_tracker.

Concept Snapshot

Morgan logs HTTP requests in Express apps.
Use app.use(morgan('format')) to add logging.
Logs show method, URL, status, and response time.
Intercepts before but logs after route handlers on response 'finish'.
Helps track and debug incoming requests easily.

Full Transcript

Morgan is a middleware for Express that logs HTTP requests. When a request arrives, Morgan intercepts it, starts a timer, passes control to the next middleware or route handler. When the response finishes ('finish' event), it formats a log line with method, URL, status, and response time, then outputs this log to the console or a file. This helps developers see request details in real time and debug their apps. The 'tiny' format is a short log style showing key info. Morgan logs on response finish, capturing accurate timing and status even if errors occur. Changing the request method changes the log output accordingly.