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

Concept Flow - Morgan for HTTP request logging

Incoming HTTP Request

↓

Morgan Middleware Intercepts

↓

Start Timer & Pass to Next

↓

Route Handler Responds

↓

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.

Practice

1. What is the main purpose of using morgan in an Express app?

easy

A. To log HTTP requests for monitoring server activity

B. To handle database connections

C. To serve static files

D. To manage user sessions

Solution

1. Step 1: Understand Morgan's role

   Morgan is a middleware that logs details about HTTP requests to the console or a file.

2. Step 2: Compare options with Morgan's function

   Only logging HTTP requests matches Morgan's purpose; other options relate to different tasks.

3. Final Answer:

   To log HTTP requests for monitoring server activity -> Option A

4. Quick Check:

   Logging HTTP requests = A [OK]

Hint: Morgan logs requests, not database or sessions [OK]

Common Mistakes:

• Confusing Morgan with session or static file middleware
• Thinking Morgan manages database connections
• Assuming Morgan serves files

2. Which of the following is the correct way to add Morgan middleware with the 'tiny' format in an Express app?

easy

A. app.use(morgan.format('tiny'))

B. app.use(morgan('tiny'))

C. app.use('morgan', 'tiny')

D. app.use(morgan.tiny())

Solution

1. Step 1: Recall Morgan usage syntax

   Morgan is used as middleware by calling morgan('format') inside app.use().

2. Step 2: Check each option's syntax

   Only app.use(morgan('tiny')) matches the correct syntax; others are invalid method calls or parameters.

3. Final Answer:

   app.use(morgan('tiny')) -> Option B

4. Quick Check:

   Use morgan('format') inside app.use() = D [OK]

Hint: Use morgan('format') inside app.use() exactly [OK]

Common Mistakes:

• Calling morgan as a method like morgan.tiny()
• Passing strings directly to app.use() without morgan()
• Using non-existent morgan.format() method

3. Given this Express code snippet, what will Morgan log when a GET request is made to '/'?

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

medium

A. GET / 200 5 - 0.123 ms

B. GET / 404 - - ms

C. POST / 200 5 - 0.123 ms

D. Error: Morgan format not found

Solution

1. Step 1: Identify the HTTP method and route

   The code handles GET requests to '/', so a GET / request is valid and returns 200 status.

2. Step 2: Understand Morgan 'dev' format output

   The 'dev' format logs method, URL, status code, response time, and response size roughly like 'GET / 200 5 - 0.123 ms'.

3. Final Answer:

   GET / 200 5 - 0.123 ms -> Option A

4. Quick Check:

   GET / with 200 status logs as in B [OK]

Hint: GET '/' with 'dev' logs method, URL, status, time [OK]

Common Mistakes:

• Confusing POST with GET method
• Assuming 404 status for existing route
• Expecting error from Morgan format

4. You added Morgan to your Express app with app.use(morgan('combined')), but no logs appear in the console. What is the most likely cause?

medium

A. Morgan requires a database connection to log

B. Morgan does not support 'combined' format

C. Morgan middleware is added after route handlers

D. Morgan only logs errors, not requests

Solution

1. Step 1: Understand middleware order importance

   Express runs middleware in order. If Morgan is added after routes, requests may not pass through it.

2. Step 2: Check Morgan format and behavior

   'combined' is a valid format; Morgan logs all requests, not just errors; it does not need a database.

3. Final Answer:

   Morgan middleware is added after route handlers -> Option C

4. Quick Check:

   Middleware order matters; Morgan must be before routes [OK]

Hint: Add Morgan before routes to see logs [OK]

Common Mistakes:

• Thinking 'combined' format is invalid
• Believing Morgan logs only errors
• Assuming Morgan needs a database

5. You want to log HTTP requests only if the response status is 400 or higher using Morgan. Which code snippet correctly achieves this?

hard

A. app.use(morgan('combined', { filter: (req, res) => res.statusCode >= 400 }))

B. app.use(morgan('combined', { skip: (req, res) => res.statusCode >= 400 }))

C. app.use(morgan('combined', { only: (req, res) => res.statusCode >= 400 }))

D. app.use(morgan('combined', { skip: (req, res) => res.statusCode < 400 }))

Solution

1. Step 1: Understand Morgan's skip option

   The 'skip' option is a function that returns true to skip logging that request.

2. Step 2: Apply condition to skip requests with status below 400

   To log only 400 or higher, skip requests where statusCode is less than 400.

3. Step 3: Verify other options

   'filter' and 'only' are not valid Morgan options; app.use(morgan('combined', { skip: (req, res) => res.statusCode >= 400 })) skips 400+ which is opposite of desired.

4. Final Answer:

   app.use(morgan('combined', { skip: (req, res) => res.statusCode < 400 })) -> Option D

5. Quick Check:

   Skip if status < 400 to log 400+ [OK]

Hint: Use skip to ignore status below 400 [OK]

Common Mistakes:

• Using skip condition backwards
• Using invalid options like filter or only
• Confusing skip with logging condition