Response handling in NestJS

Introduction

Response handling lets your server send back clear and useful answers to users or other programs. It helps show the right data or messages after someone asks for something.

When you want to send data back after a user asks for information.

When you need to send a success or error message after a user sends data.

When you want to control the status code and headers of the reply.

When you want to format the response in JSON or other formats.

When you want to add custom headers or cookies to the response.

Syntax

NestJS

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

@Controller('example')
export class ExampleController {
  @Get()
  getExample(@Res() res: Response) {
    res.status(200).json({ message: 'Hello World' });
  }
}

Use @Res() decorator to get the raw response object from Express.

Call methods like status() and json() on the response to send data.

Examples

Sends a plain text response with status 200.

NestJS

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

@Controller('hello')
export class HelloController {
  @Get()
  sayHello(@Res() res: Response) {
    res.status(200).send('Hello NestJS');
  }
}

Sends a JSON response with status 201.

NestJS

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

@Controller('json')
export class JsonController {
  @Get()
  sendJson(@Res() res: Response) {
    res.status(201).json({ success: true, data: [1, 2, 3] });
  }
}

Returns data directly without using @Res(). NestJS handles the response automatically.

NestJS

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

@Controller('simple')
export class SimpleController {
  @Get()
  getSimple() {
    return { message: 'Auto handled response' };
  }
}

Sample Program

This controller sends a JSON greeting message with status code 200 when you visit the '/greet' route.

NestJS

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

@Controller('greet')
export class GreetController {
  @Get()
  greetUser(@Res() res: Response) {
    const greeting = { message: 'Welcome to NestJS!' };
    res.status(200).json(greeting);
  }
}

Important Notes

Using @Res() means you must handle sending the response yourself.

If you return data directly without @Res(), NestJS sends the response automatically.

Always set the correct status code to help clients understand the result.

Summary

Response handling controls what your server sends back to users or clients.

You can use @Res() to customize the response fully or return data directly for automatic handling.

Setting status codes and response formats clearly improves communication with clients.

Practice

1. In NestJS, what is the primary purpose of using the @Res() decorator in a controller method?

easy

A. To define a route path for the controller method

B. To inject a service into the controller

C. To access and customize the HTTP response object directly

D. To automatically serialize the response data

Solution

1. Step 1: Understand the role of @Res()

   The @Res() decorator gives direct access to the HTTP response object, allowing full control over the response.

2. Step 2: Differentiate from other decorators

   Other decorators like @Inject() inject services, and route decorators define paths, but @Res() is specifically for response handling.

3. Final Answer:

   To access and customize the HTTP response object directly -> Option C

4. Quick Check:

   @Res() = direct response control [OK]

Hint: Remember: @Res() means direct response access [OK]

Common Mistakes:

• Confusing @Res() with service injection
• Thinking @Res() defines routes
• Assuming @Res() auto-serializes data

2. Which of the following is the correct way to set a 201 status code and send a JSON response using @Res() in NestJS?

easy

A. res.status(201).json({ message: 'Created' });

B. res.sendStatus(201).json({ message: 'Created' });

C. res.json(201, { message: 'Created' });

D. res.statusCode(201).send({ message: 'Created' });

Solution

1. Step 1: Recall Express response methods

   In NestJS, @Res() uses Express response methods. The correct way to set status and send JSON is res.status(201).json(...).

2. Step 2: Identify incorrect method usage

   sendStatus sends status and ends response, so chaining .json() is invalid. res.json(201, ...) and res.statusCode(201) are not valid Express methods.

3. Final Answer:

   res.status(201).json({ message: 'Created' }); -> Option A

4. Quick Check:

   Set status then json = res.status(201).json(...) [OK]

Hint: Use res.status(code).json(data) to set status and send JSON [OK]

Common Mistakes:

• Using sendStatus then chaining json
• Passing status code as first json argument
• Using non-existent statusCode method

3. Given the following NestJS controller method, what will be the HTTP status code and response body sent to the client?

async getData(@Res() res) {
  const data = { id: 1, name: 'NestJS' };
  return res.status(202).json(data);
}

medium

A. Status 202 with body { id: 1, name: 'NestJS' }

B. Status 200 with body { id: 1, name: 'NestJS' }

C. Status 202 with empty body

D. Status 500 with error message

Solution

1. Step 1: Analyze response status setting

   The method calls res.status(202), so the HTTP status code will be 202 (Accepted).

2. Step 2: Analyze response body sent

   The method sends JSON data { id: 1, name: 'NestJS' } with .json(data), so the body will contain this object.

3. Final Answer:

   Status 202 with body { id: 1, name: 'NestJS' } -> Option A

4. Quick Check:

   Status 202 + JSON data = Status 202 with body { id: 1, name: 'NestJS' } [OK]

Hint: res.status(code).json(data) sets status and sends data [OK]

Common Mistakes:

• Assuming default 200 status instead of 202
• Thinking body is empty when using json()
• Confusing status codes with error codes

4. Identify the error in the following NestJS controller method that uses @Res():

async sendResponse(@Res() res) {
  res.json({ message: 'Hello' });
  res.status(200);
}

medium

A. res.status must be called before returning from method

B. Missing await keyword before res.json

C. res.json should be replaced with res.sendStatus

D. Calling res.status after res.json causes no effect

Solution

1. Step 1: Understand Express response flow

   Calling res.json() sends the response immediately and ends it.

2. Step 2: Analyze order of method calls

   Calling res.status(200) after res.json() has no effect because response is already sent.

3. Final Answer:

   Calling res.status after res.json causes no effect -> Option D

4. Quick Check:

   Set status before sending response [OK]

Hint: Always set status before sending response [OK]

Common Mistakes:

• Calling res.status after res.json
• Thinking res.json needs await
• Confusing res.sendStatus with res.json

5. You want to create a NestJS controller method that returns a 404 status with a JSON error message if a user is not found, but returns the user data with status 200 if found. Which code snippet correctly implements this response handling?

hard

A. async getUser(@Res() res, @Param('id') id: string) {
const user = await this.userService.findById(id);
if (!user) {
res.json({ error: 'User not found' }).status(404);
} else {
res.json(user).status(200);
}
}

B. async getUser(@Res() res, @Param('id') id: string) {
const user = await this.userService.findById(id);
if (!user) {
return res.status(404).json({ error: 'User not found' });
}
return res.status(200).json(user);
}

C. async getUser(@Res() res, @Param('id') id: string) {
const user = await this.userService.findById(id);
if (!user) {
res.json({ error: 'User not found' });
res.status(404);
} else {
res.json(user);
res.status(200);
}
}

D. async getUser(@Res() res, @Param('id') id: string) {
const user = await this.userService.findById(id);
if (!user) {
res.sendStatus(404);
} else {
res.json(user);
}
}

Solution

1. Step 1: Check correct chaining of status and json

   async getUser(@Res() res, @Param('id') id: string) { const user = await this.userService.findById(id); if (!user) { return res.status(404).json({ error: 'User not found' }); } return res.status(200).json(user); } correctly calls res.status(404).json(...) and res.status(200).json(...), chaining status and JSON response properly.

2. Step 2: Identify errors in other options

   async getUser(@Res() res, @Param('id') id: string) { const user = await this.userService.findById(id); if (!user) { res.json({ error: 'User not found' }).status(404); } else { res.json(user).status(200); } } calls res.json() before res.status(), which does not set status correctly. async getUser(@Res() res, @Param('id') id: string) { const user = await this.userService.findById(id); if (!user) { res.json({ error: 'User not found' }); res.status(404); } else { res.json(user); res.status(200); } } calls res.json() before res.status(), which does not set status correctly. async getUser(@Res() res, @Param('id') id: string) { const user = await this.userService.findById(id); if (!user) { res.sendStatus(404); } else { res.json(user); } } uses res.sendStatus(404) which sends response immediately but does not send JSON error message.

3. Final Answer:

   correctly sets status and sends JSON error or data -> Option B

4. Quick Check:

   Chain status and json for correct response [OK]

Hint: Chain res.status(code).json(data) for clear responses [OK]

Common Mistakes:

• Calling res.json before res.status
• Using res.sendStatus without JSON body
• Separating status and json calls without chaining