Status codes and headers in NestJS - Step-by-Step Execution

Concept Flow - Status codes and headers

Receive HTTP Request

↓

Controller handles request

↓

Set Status Code

↓

Set Headers

↓

Send Response with Body

↓

Client receives response

The server receives a request, the controller sets status code and headers, then sends the response back to the client.

Execution Sample

NestJS

import { Controller, Get, Res } from '@nestjs/common';

@Controller('example')
export class ExampleController {
  @Get()
  getExample(@Res() res) {
    res.status(201).setHeader('X-Custom-Header', 'NestJS').send('Created');
  }
}

This code sends a response with status 201 and a custom header 'X-Custom-Header'.

Execution Table

Step	Action	Status Code	Headers Set	Response Body	Effect
1	Receive GET request at /example	N/A	None	None	Start processing request
2	Controller method getExample called	N/A	None	None	Prepare response
3	Set status code to 201	201	None	None	Response status set to Created
4	Set header 'X-Custom-Header' to 'NestJS'	201	X-Custom-Header: NestJS	None	Custom header added
5	Send response body 'Created'	201	X-Custom-Header: NestJS	Created	Response sent to client
6	Client receives response	201	X-Custom-Header: NestJS	Created	Request complete

💡 Response sent with status 201, header set, and body 'Created'

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
statusCode	undefined	201	201	201	201
headers	{}	{}	{'X-Custom-Header': 'NestJS'}	{'X-Custom-Header': 'NestJS'}	{'X-Custom-Header': 'NestJS'}
responseBody	undefined	undefined	undefined	'Created'	'Created'

Key Moments - 3 Insights

Why do we set the status code before sending the response body?

The status code must be set before sending the response because once the body is sent, the response is finalized and headers or status cannot be changed. See execution_table step 3 and 5.

Can we set multiple headers before sending the response?

Yes, you can set multiple headers before sending the response. Each header is added before the send call. In the example, only one header is set at step 4, but more can be added similarly.

What happens if we try to set headers after sending the response?

Headers cannot be modified after the response is sent. Attempting to do so will have no effect or cause an error. The response is finalized at step 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the status code at step 4?

A 201

B 200

C undefined

D 404

💡 Hint

Check the 'Status Code' column at step 4 in the execution_table.

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

A Step 4

B Step 5

C Step 3

D Step 6

💡 Hint

Look for the step where 'Response Body' changes from undefined to 'Created' in the execution_table.

If we add another header before sending, how would the headers look at step 4?

A {}

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

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

D Headers cannot be changed

💡 Hint

Refer to variable_tracker headers row after step 4 and imagine adding one more header.

Concept Snapshot

In NestJS controllers, use res.status(code) to set HTTP status.
Use res.setHeader(name, value) to add headers.
Call res.send(body) last to send response.
Set status and headers before sending.
This controls how clients understand the response.

Full Transcript

In NestJS, when a controller receives a request, it prepares a response by setting a status code and headers before sending the response body. The status code tells the client if the request was successful or not, like 201 for created. Headers add extra info, like custom metadata. The response body is sent last, finalizing the response. Trying to change status or headers after sending won't work. This flow ensures clients get clear, complete responses.