Response handling in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the HTTP status code returned by this NestJS controller method?

Consider the following NestJS controller method. What HTTP status code will the client receive when this endpoint is called?

NestJS

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

@Controller('items')
export class ItemsController {
  @Get()
  getItems(@Res() res: Response) {
    res.json({ message: 'List of items' });
  }
}

A 200 OK

B 201 Created

C 204 No Content

D 500 Internal Server Error

💡 Hint

When you use res.json() without setting status, what is the default HTTP status code?

Using res.json() sends a JSON response with status code 200 by default.

📝 Syntax

intermediate

Which option correctly sets a custom HTTP header in a NestJS response?

You want to add a custom header 'X-Custom-Header' with value 'NestJS' to the HTTP response. Which code snippet correctly does this?

NestJS

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

@Controller('test')
export class TestController {
  @Get('header')
  setHeader(@Res() res: Response) {
    // Add custom header here
    res.send('Header set');
  }
}

A res.append('X-Custom-Header', 'NestJS');

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

C res.addHeader('X-Custom-Header', 'NestJS');

D res.header('X-Custom-Header', 'NestJS');

💡 Hint

Check Express response methods for setting headers.

The res.set() method sets HTTP headers correctly. res.header() is an alias but not recommended in NestJS context. res.addHeader() does not exist. res.append() appends values to existing headers.

❓ state_output

advanced

What will be the JSON response body from this NestJS controller method?

Analyze the following NestJS controller method. What JSON will the client receive?

NestJS

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

@Controller('users')
export class UsersController {
  @Get('profile')
  getProfile() {
    return { id: 1, name: 'Alice', active: true };
  }
}

A {"id":1,"name":"Alice","active":true}

B "{id:1, name:'Alice', active:true}"

C {"id":"1","name":"Alice","active":"true"}

D {"id":1,"name":"Alice"}

💡 Hint

What does returning a plain object from a NestJS controller method do?

Returning a plain object from a controller method sends it as JSON with correct types preserved.

🔧 Debug

advanced

Why does this NestJS controller method cause a runtime error?

Examine the code below. When calling this endpoint, a runtime error occurs. What is the cause?

NestJS

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

@Controller('orders')
export class OrdersController {
  @Get('list')
  listOrders(@Res() res: Response) {
    res.json({ orders: [] });
  }
}

A res.json() is not a function on Response object.

B Missing await keyword before res.json() causes error.

C Using both return and res.json() causes NestJS to send response twice.

D The method lacks a @Post decorator causing mismatch.

💡 Hint

Check how NestJS handles responses when @Res() is used with return statements.

When using @Res() and calling res methods directly, you should not return a value. Returning causes NestJS to try sending response again, leading to an error.

🧠 Conceptual

expert

Which option correctly describes how NestJS handles response serialization by default?

In NestJS, when a controller method returns a plain object, how does NestJS handle the response serialization and what can you do to customize it?

A NestJS requires you to always use @Res() and call res.json() to send JSON responses.

B NestJS sends returned objects as raw JavaScript without serialization; you must manually call JSON.stringify().

C NestJS converts returned objects to XML by default; JSON serialization requires extra setup.

D NestJS automatically serializes returned objects to JSON using class-transformer; you can customize serialization with @Exclude and @Expose decorators.

💡 Hint

Think about how NestJS integrates with class-transformer for response objects.

By default, NestJS uses class-transformer to serialize returned objects to JSON. You can control which properties are included using decorators like @Exclude and @Expose.