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.

Practice

1. In NestJS, what does the HTTP status code 201 usually indicate in a response?

easy

A. A new resource was successfully created

B. The request was malformed or invalid

C. The requested resource was not found

D. The server encountered an internal error

Solution

1. Step 1: Understand HTTP status codes

   HTTP status codes communicate the result of a client's request. Code 201 means "Created".

2. Step 2: Match 201 with its meaning

   201 specifically means a new resource was successfully created on the server.

3. Final Answer:

   A new resource was successfully created -> Option A

4. Quick Check:

   201 = Created [OK]

Hint: 201 means resource created successfully [OK]

Common Mistakes:

• Confusing 201 with 404 (not found)
• Thinking 201 means server error
• Mixing 201 with 400 (bad request)

2. Which of the following is the correct way to set a custom header X-Custom-Header with value 123 in a NestJS controller using Express response?

easy

A. res.setHeader('X-Custom-Header', 789);

B. res.set('X-Custom-Header', '123');

C. res.header('X-Custom-Header', '789');

D. res.addHeader('X-Custom-Header', '123');

Solution

1. Step 1: Recall Express response methods

   Express response object uses res.set() to set headers.

2. Step 2: Check method and value types

   res.set() accepts header name and string value. res.set('X-Custom-Header', '123'); uses correct method and string value.

3. Final Answer:

   res.set('X-Custom-Header', '123'); -> Option B

4. Quick Check:

   Use res.set() to set headers [OK]

Hint: Use res.set() to set headers in NestJS [OK]

Common Mistakes:

• Using res.setHeader (not Express method)
• Passing number instead of string as header value
• Using non-existent res.addHeader method

3. What will be the HTTP status code and header in the response when this NestJS controller method runs?

async getData(@Res() res) {
  res.status(204).set('X-Info', 'No Content').send();
}

medium

A. Status 204 with no headers

B. Status 200 with header X-Info: No Content

C. Status 204 with header X-Info: No Content

D. Status 404 with header X-Info: No Content

Solution

1. Step 1: Analyze status and header setting

   The code sets status to 204 and header 'X-Info' to 'No Content'.

2. Step 2: Understand 204 status behavior

   Status 204 means "No Content" and allows custom headers. The header is set correctly.

3. Final Answer:

   Status 204 with header X-Info: No Content -> Option C

4. Quick Check:

   Status 204 + header set = Status 204 with header X-Info: No Content [OK]

Hint: res.status(204).set() sets status and headers [OK]

Common Mistakes:

• Assuming 204 means no headers allowed
• Confusing default status 200 with explicit 204
• Ignoring header set after status

4. Identify the error in this NestJS controller method that tries to set status and headers:

async update(@Res() res) {
  res.status(200);
  res.set('Content-Type', 'application/json');
  res.send({ success: true });
  res.status(201);
}

medium

A. Calling res.status(201) after res.send() has no effect

B. res.set() must be called before res.status()

C. res.send() cannot send JSON objects

D. res.status() cannot be called twice

Solution

1. Step 1: Review response flow

   res.send() sends the response and ends it. Any code after it won't affect response.

2. Step 2: Check status calls order

   Calling res.status(201) after res.send() does nothing because response is already sent.

3. Final Answer:

   Calling res.status(201) after res.send() has no effect -> Option A

4. Quick Check:

   res.send() ends response, later status calls ignored [OK]

Hint: Set status before res.send(), not after [OK]

Common Mistakes:

• Trying to set status after sending response
• Thinking res.send() can't send JSON
• Believing res.set() must come before res.status()

5. You want to send a 202 Accepted status with a custom header X-Request-ID and a JSON body in NestJS. Which code snippet correctly achieves this?

hard

A. res.send({ message: 'Accepted' }).status(202).set('X-Request-ID', requestId);

B. res.set('X-Request-ID', requestId).status(202).send('Accepted');

C. res.status(202).send({ message: 'Accepted' }).set('X-Request-ID', requestId);

D. res.status(202).set('X-Request-ID', requestId).json({ message: 'Accepted' });

Solution

1. Step 1: Order of method calls matters

   In Express, chaining status, set, and json works if status is set before sending.

2. Step 2: Check each option's correctness

   Options A and C call send before completing status/header setup, invalid. res.set('X-Request-ID', requestId).status(202).send('Accepted'); sets header and status but uses res.send('Accepted') sending plain text instead of JSON object. res.status(202).set('X-Request-ID', requestId).json({ message: 'Accepted' }); correctly sets status, header, sends JSON.

3. Step 3: Confirm best practice

   res.status(202).set('X-Request-ID', requestId).json({ message: 'Accepted' }); is the clearest and most common pattern: status, set header, then send JSON.

4. Final Answer:

   res.status(202).set('X-Request-ID', requestId).json({ message: 'Accepted' }); -> Option D

5. Quick Check:

   Status then headers then send JSON [OK]

Hint: Chain status, set, then send/json in that order [OK]

Common Mistakes:

• Calling send before setting status or headers
• Using send instead of json for JSON body
• Setting headers after sending response