Routing requests manually in Node.js - Step-by-Step Execution

Concept Flow - Routing requests manually

Start Server

↓

Receive HTTP Request

↓

Check Request URL

↓

Handle1

↓

Send Response

↓

End

The server starts and waits for requests. When a request comes, it checks the URL path and sends the matching response or a 404 if no match.

Execution Sample

Node.js

const http = require('http');
const server = http.createServer((req, res) => {
  if (req.url === '/hello') {
    res.end('Hello World');
  } else {
    res.statusCode = 404; res.end('Not Found');
  }
});
server.listen(3000);

This code creates a server that responds with 'Hello World' for '/hello' path and 'Not Found' for others.

Execution Table

Step	Request URL	Condition Checked	Condition Result	Action Taken	Response Sent
1	/hello	req.url === '/hello'	true	Send 'Hello World'	'Hello World'
2	/goodbye	req.url === '/hello'	false	Set status 404 and send 'Not Found'	'Not Found'
3	/unknown	req.url === '/hello'	false	Set status 404 and send 'Not Found'	'Not Found'

💡 Requests handled based on URL; unmatched URLs return 404 Not Found.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
req.url	undefined	/hello	/goodbye	/unknown
res.statusCode	200 (default)	200	404	404
res content	empty	'Hello World'	'Not Found'	'Not Found'

Key Moments - 2 Insights

Why does the server send 'Not Found' for URLs other than '/hello'?

Because the condition req.url === '/hello' is false for other URLs, so the else block sets statusCode to 404 and sends 'Not Found' as shown in execution_table rows 2 and 3.

What happens if we forget to call res.end()?

The server will not send the response back properly, causing the client to wait indefinitely. In the code, res.end() finalizes the response, so missing it stops the response from completing.

Visual Quiz - 3 Questions

Look at the execution table, what response is sent when the request URL is '/hello'?

A 'Not Found'

B 'Hello World'

C Empty response

D Error message

💡 Hint

Check the 'Response Sent' column for Step 1 in the execution_table.

At which step does the server set the status code to 404?

A Both Step 2 and Step 3

B Step 1

C Step 3

D Step 2

💡 Hint

Look at the 'Action Taken' and 'res.statusCode' in variable_tracker for Steps 2 and 3.

If we add a new condition for '/goodbye' to send 'Goodbye!', how would the execution table change for Step 2?

A Server crashes

B Condition result remains false and response is 'Not Found'

C Condition result becomes true and response is 'Goodbye!'

D Response is empty

💡 Hint

Think about adding a new if block for '/goodbye' before the else, changing Step 2's action.

Concept Snapshot

Routing requests manually in Node.js:
- Create server with http.createServer
- Check req.url in callback
- Use if/else to match paths
- Send response with res.end()
- Set res.statusCode for errors
- Unmatched paths return 404 Not Found

Full Transcript

This example shows how to handle HTTP requests manually in Node.js. The server listens for requests and checks the URL path. If the path is '/hello', it sends back 'Hello World'. For any other path, it sets the status code to 404 and sends 'Not Found'. The key is to check req.url and respond accordingly. The response must be ended with res.end() to complete the request. This manual routing is the basic way servers decide what to send back based on the URL.

Practice

1. What is the main purpose of checking req.url in a Node.js server without frameworks?

easy

A. To decide how to respond to different request paths

B. To parse the request body automatically

C. To connect to a database

D. To handle user authentication

Solution

1. Step 1: Understand the role of req.url

   req.url contains the path requested by the client, like '/home' or '/about'.

2. Step 2: Purpose of checking req.url

   By checking req.url, the server can decide which content or response to send back.

3. Final Answer:

   To decide how to respond to different request paths -> Option A

4. Quick Check:

   Routing = Check req.url [OK]

Hint: Check req.url to route requests manually [OK]

Common Mistakes:

• Thinking req.url parses request body
• Confusing req.url with database connection
• Assuming req.url handles authentication

2. Which of the following is the correct way to send a plain text response with status 200 in a manual Node.js server?

easy

A. res.status(200).send('Hello');

B. res.writeHead(200, {'Content-Type': 'text/plain'}); res.end('Hello');

C. res.sendStatus(200).send('Hello');

D. res.write(200, 'Hello');

Solution

1. Step 1: Identify correct methods for manual response

   In manual Node.js servers, res.writeHead() sets status and headers, res.end() sends the response body.

2. Step 2: Check each option

   res.writeHead(200, {'Content-Type': 'text/plain'}); res.end('Hello'); uses writeHead and end correctly. The second and third options use Express methods, not native Node.js. res.write(200, 'Hello'); is invalid syntax.

3. Final Answer:

   res.writeHead(200, {'Content-Type': 'text/plain'}); res.end('Hello'); -> Option B

4. Quick Check:

   Manual response = writeHead + end [OK]

Hint: Use writeHead and end to send manual responses [OK]

Common Mistakes:

• Using Express methods in plain Node.js
• Calling res.write without headers
• Incorrect method names like res.sendStatus

3. What will the following Node.js server code output when a client requests /hello?

const http = require('http');
const server = http.createServer((req, res) => {
  if (req.url === '/hello') {
    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.end('Hello World');
  } else {
    res.writeHead(404);
    res.end('Not Found');
  }
});
server.listen(3000);

medium

A. Error: Cannot read property 'writeHead' of undefined

B. Not Found

C. Hello World

D. Server crashes

Solution

1. Step 1: Check the request URL condition

   The code checks if req.url === '/hello'. If true, it sends 'Hello World' with status 200.

2. Step 2: Determine output for '/hello'

   Since the request is '/hello', the first branch runs, sending 'Hello World'.

3. Final Answer:

   Hello World -> Option C

4. Quick Check:

   req.url '/hello' = 'Hello World' [OK]

Hint: Match req.url exactly to send correct response [OK]

Common Mistakes:

• Assuming default response is 'Hello World'
• Confusing 404 and 200 responses
• Expecting server error without syntax issues

4. Identify the error in this manual routing code snippet:

const http = require('http');
const server = http.createServer((req, res) => {
  if (req.url = '/test') {
    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.end('Test page');
  } else {
    res.writeHead(404);
    res.end('Page not found');
  }
});
server.listen(3000);

medium

A. Missing res.end() call

B. server.listen() missing port number

C. Incorrect header format in writeHead

D. Using assignment (=) instead of comparison (===) in if condition

Solution

1. Step 1: Check the if condition syntax

   The condition uses req.url = '/test', which assigns instead of compares. This always evaluates to true.

2. Step 2: Identify correct comparison operator

   It should be req.url === '/test' to compare values properly.

3. Final Answer:

   Using assignment (=) instead of comparison (===) in if condition -> Option D

4. Quick Check:

   Use === for comparison, not = [OK]

Hint: Use === for comparison, not = [OK]

Common Mistakes:

• Confusing assignment and comparison operators
• Forgetting to call res.end()
• Incorrect header object syntax

5. You want to manually route requests to serve JSON data at /data and plain text at /info. Which code snippet correctly implements this?

hard

A. if (req.url === '/data') { res.writeHead(200, {'Content-Type': 'application/json'}); res.end(JSON.stringify({name: 'Node'})); } else if (req.url === '/info') { res.writeHead(200, {'Content-Type': 'text/plain'}); res.end('Information'); } else { res.writeHead(404); res.end('Not Found'); }

B. if (req.url === '/data') { res.end({name: 'Node'}); } else if (req.url === '/info') { res.end('Information'); } else { res.end('Not Found'); }

C. if (req.url === '/data') { res.writeHead(200, {'Content-Type': 'text/plain'}); res.end('{"name":"Node"}'); } else if (req.url === '/info') { res.writeHead(200, {'Content-Type': 'application/json'}); res.end('Information'); } else { res.writeHead(404); res.end('Not Found'); }

D. if (req.url === '/data') { res.writeHead(404); res.end('Not Found'); } else if (req.url === '/info') { res.writeHead(200); res.end('Information'); }

Solution

1. Step 1: Check content types for each route

   For /data, JSON content type 'application/json' is correct. For /info, plain text 'text/plain' is correct.

2. Step 2: Verify response body and headers

   if (req.url === '/data') { res.writeHead(200, {'Content-Type': 'application/json'}); res.end(JSON.stringify({name: 'Node'})); } else if (req.url === '/info') { res.writeHead(200, {'Content-Type': 'text/plain'}); res.end('Information'); } else { res.writeHead(404); res.end('Not Found'); } sets correct headers and uses JSON.stringify for JSON data. Other options misuse content types or omit headers.

3. Final Answer:

   correctly sets headers and responses for both routes -> Option A

4. Quick Check:

   Set correct Content-Type and stringify JSON [OK]

Hint: Set Content-Type and stringify JSON for JSON responses [OK]

Common Mistakes:

• Sending JSON object directly without stringify
• Mixing content types for routes
• Omitting status codes or headers