Setting response headers in Node.js - Step-by-Step Execution

Concept Flow - Setting response headers

Start HTTP request

↓

Create response object

↓

Set response headers

↓

Write response body

↓

Send response to client

↓

End request handling

This flow shows how a Node.js server sets headers on the response before sending it back to the client.

Execution Sample

Node.js

const http = require('http');
const server = http.createServer((req, res) => {
  res.setHeader('Content-Type', 'text/plain');
  res.setHeader('X-Custom-Header', 'MyValue');
  res.end('Hello World');
});
server.listen(3000);

This code creates a simple HTTP server that sets two headers before sending a plain text response.

Execution Table

Step	Action	Header Set	Header Value	Response State
1	Start request handling	-	-	No headers set
2	Set header	Content-Type	text/plain	Headers: {Content-Type: text/plain}
3	Set header	X-Custom-Header	MyValue	Headers: {Content-Type: text/plain, X-Custom-Header: MyValue}
4	Write and send response	-	-	Response sent with headers and body
5	End request handling	-	-	Request complete

💡 Response sent and request handling finished

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
res.headers	{}	{Content-Type: 'text/plain'}	{Content-Type: 'text/plain', X-Custom-Header: 'MyValue'}	{Content-Type: 'text/plain', X-Custom-Header: 'MyValue'}

Key Moments - 2 Insights

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

Headers must be set before sending the response body because once res.end() is called, the response is sent and headers cannot be changed. See execution_table steps 3 and 4.

What happens if we set the same header twice?

Setting the same header twice overwrites the previous value. Only the last value is sent. This is why each header key appears once in res.headers as shown in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution table, what headers are set after step 3?

A {}

B {Content-Type: 'text/html'}

C {Content-Type: 'text/plain', X-Custom-Header: 'MyValue'}

D {X-Custom-Header: 'MyValue'}

💡 Hint

Check the 'Header Set' and 'Response State' columns at step 3 in the execution_table.

At which step is the response body sent to the client?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look for the action 'Write and send response' in the execution_table.

If we set a header after calling res.end(), what happens?

A The header is added successfully

B The header is ignored because response is already sent

C The header overwrites previous headers

D The server crashes

💡 Hint

Refer to key_moments about why headers must be set before res.end() and execution_table steps 4 and 5.

Concept Snapshot

Setting response headers in Node.js:
- Use res.setHeader(name, value) before res.end()
- Headers define metadata like content type
- Setting headers after res.end() has no effect
- Overwriting headers replaces previous values
- Always set all headers before sending response body

Full Transcript

This example shows how a Node.js HTTP server sets response headers before sending the response. The server starts handling a request, sets the Content-Type and a custom header, then sends the response body with res.end(). Headers must be set before calling res.end() because after that the response is sent and headers cannot be changed. Setting the same header twice overwrites the previous value. The execution table tracks each step, showing headers added and when the response is sent. The variable tracker shows how the headers object changes after each setHeader call. This helps beginners understand the order and effect of setting headers in Node.js.