CRUD operations in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this NestJS controller method?

Consider this NestJS controller method that handles a GET request to fetch all items. What will be the HTTP response body when this method is called?

NestJS

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

@Controller('items')
export class ItemsController {
  @Get()
  findAll() {
    return ['apple', 'banana', 'cherry'];
  }
}

A HTTP 200 with JSON body: ["apple", "banana", "cherry"]

B HTTP 500 Internal Server Error

C HTTP 404 Not Found error

D HTTP 200 with empty JSON array []

💡 Hint

The method returns a simple array directly.

The controller method returns an array, so NestJS sends it as JSON with status 200.

❓ state_output

intermediate

What is the value of the 'items' array after these service calls?

Given this NestJS service managing an in-memory list, what is the content of the 'items' array after executing the code below?

NestJS

export class ItemsService {
  private items = ['a', 'b'];

  addItem(item: string) {
    this.items.push(item);
  }

  removeItem(item: string) {
    this.items = this.items.filter(i => i !== item);
  }

  getItems() {
    return this.items;
  }
}

const service = new ItemsService();
service.addItem('c');
service.removeItem('a');
const result = service.getItems();

A ["a", "b", "c"]

B ["c"]

C ["a", "b"]

D ["b", "c"]

💡 Hint

Adding 'c' then removing 'a' affects the array.

Initially ['a','b'], add 'c' → ['a','b','c'], remove 'a' → ['b','c'].

📝 Syntax

advanced

Which option causes a syntax error in this NestJS controller method?

Identify the option that will cause a syntax error when defining a POST method in a NestJS controller.

NestJS

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

@Controller('users')
export class UsersController {
  @Post()
  createUser(@Body() userData) {
    return userData;
  }
}

A

}
;ataDresu nruter  
{ )ataDresu )(ydoB@(resUetaerc
)(tsoP@

B

@Post()
createUser(@Body() userData) {
  return userData;
}

C

@Post()
createUser(@Body userData) {
  return userData;
}

D

@Post()
createUser(@Body() userData) {
  return userData
}

💡 Hint

Check the decorator syntax carefully.

The @Body decorator requires parentheses when used as a decorator. Missing () causes syntax error.

🔧 Debug

advanced

Why does this NestJS service method throw a runtime error?

This service method is supposed to update an item by id. Why does it throw an error when called?

NestJS

export class ItemsService {
  private items = [{ id: 1, name: 'a' }, { id: 2, name: 'b' }];

  updateItem(id: number, newName: string) {
    const item = this.items.find(i => i.id === id);
    item.name = newName;
  }
}

const service = new ItemsService();
service.updateItem(3, 'c');

A ReferenceError: item is not defined

B TypeError: Cannot set property 'name' of undefined

C No error, item updated successfully

D SyntaxError: Unexpected token '='

💡 Hint

What happens if find() returns undefined?

If no item matches id, find() returns undefined, so item.name causes TypeError.

🧠 Conceptual

expert

Which option correctly describes the role of DTOs in NestJS CRUD operations?

In NestJS, Data Transfer Objects (DTOs) are often used in CRUD operations. Which statement best describes their purpose?

A DTOs define the shape of data sent over the network and help validate input before processing.

B DTOs are used to directly access the database without services.

C DTOs automatically generate database tables based on controller methods.

D DTOs replace controllers and handle HTTP requests themselves.

💡 Hint

Think about how data is structured and validated in NestJS.

DTOs specify data structure and validation rules to ensure safe and consistent data handling.