Status code usage patterns in Node.js - Step-by-Step Execution

Concept Flow - Status code usage patterns

Receive HTTP Request

↓

Process Request Logic

↓

Determine Status Code

↓

Set Response Status Code

↓

Send Response with Status Code

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Client Receives Response

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Client Handles Based on Status Code

This flow shows how a Node.js server receives a request, decides the right status code, sends it back, and how the client uses it.

Execution Sample

Node.js

import http from 'http';

const server = http.createServer((req, res) => {
  if (req.url === '/success') {
    res.statusCode = 200;
    res.end('OK');
  } else {
    res.statusCode = 404;
    res.end('Not Found');
  }
});

server.listen(3000);

A simple Node.js server that sends 200 for '/success' and 404 for other URLs.

Execution Table

Step	Request URL	Condition	Status Code Set	Response Sent
1	/success	req.url === '/success' is true	200	'OK'
2	/unknown	req.url === '/success' is false	404	'Not Found'
3	-	No more requests	-	-

💡 No more requests to process, server keeps running.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
req.url	undefined	/success	/unknown	varies per request
res.statusCode	200 (default)	200	404	varies per request
res.end()	not called	called with 'OK'	called with 'Not Found'	called per request

Key Moments - 3 Insights

Why do we set res.statusCode before calling res.end()?

Because the status code must be set before sending the response body; see execution_table rows 1 and 2 where statusCode is set before res.end() is called.

What happens if we don't set a status code explicitly?

Node.js defaults to 200 OK if no status code is set; this is why res.statusCode starts at 200 in variable_tracker.

Why does the server send 404 for unknown URLs?

Because the condition req.url === '/success' is false, so the else branch sets statusCode to 404 and sends 'Not Found' as shown in execution_table row 2.

Visual Quiz - 3 Questions

Look at the execution_table, what status code is sent when the request URL is '/success'?

A 404

B 500

C 200

D 301

💡 Hint

Check execution_table row 1 under 'Status Code Set'

At which step does the server send a 'Not Found' response?

A Step 1

B Step 2

C Step 3

D No step sends 'Not Found'

💡 Hint

Look at execution_table row 2 under 'Response Sent'

If we add a new URL '/error' that sets statusCode to 500, how would the execution_table change?

A Add a new row with statusCode 500 and appropriate response

B Change step 1 statusCode to 500

C Remove step 2

D No change needed

💡 Hint

New request URLs add new rows as seen in execution_table rows 1 and 2

Concept Snapshot

Status code usage in Node.js:
- Set res.statusCode before res.end()
- 200 means success
- 404 means resource not found
- Use status codes to tell client what happened
- Always send a response to end request
- Status codes guide client behavior

Full Transcript

This visual execution shows how a Node.js server handles HTTP requests by checking the URL, setting the right status code, and sending a response. When the URL is '/success', the server sets status code 200 and sends 'OK'. For other URLs, it sets 404 and sends 'Not Found'. The status code must be set before sending the response body. If no status code is set, Node.js defaults to 200. This pattern helps clients understand if their request succeeded or failed. The execution table tracks each step, showing the request URL, condition checked, status code set, and response sent. Variable tracker shows how request URL and response status code change per request. Key moments clarify why order matters and why 404 is sent for unknown URLs. The quiz tests understanding of status codes sent at each step and how adding new URLs affects the flow.