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.

Practice

1. In NestJS, which decorator is used in a controller method to handle HTTP POST requests for creating data?

easy

A. @Post

B. @Get

C. @Put

D. @Delete

Solution

1. Step 1: Understand HTTP methods and their NestJS decorators

   HTTP POST is used to create new data. NestJS uses @Post decorator for this.

2. Step 2: Match the decorator to the HTTP method

   @Get is for reading, @Put for updating, and @Delete for deleting data, so they don't fit creating data.

3. Final Answer:

   @Post -> Option A

4. Quick Check:

   POST = @Post [OK]

Hint: POST requests use @Post decorator in NestJS controllers [OK]

Common Mistakes:

• Confusing @Get with @Post
• Using @Put for creating data
• Using @Delete for creation

2. Which of the following is the correct way to extract a route parameter named id in a NestJS controller method?

easy

A. async getItem(@Body('id') id: string) {}

B. async getItem(@Query('id') id: string) {}

C. async getItem(@Param('id') id: string) {}

D. async getItem(@Headers('id') id: string) {}

Solution

1. Step 1: Identify the decorator for route parameters

   @Param is used to extract parameters from the URL path, such as /items/:id.

2. Step 2: Check other decorators and their uses

   @Body extracts data from the request body, @Query from query strings, and @Headers from HTTP headers, so they don't get route params.

3. Final Answer:

   async getItem(@Param('id') id: string) {} -> Option C

4. Quick Check:

   Route param = @Param [OK]

Hint: Use @Param to get URL parameters in NestJS [OK]

Common Mistakes:

• Using @Body to get URL params
• Using @Query for route params
• Confusing @Headers with @Param

3. Given this NestJS controller method, what will be the response if a client sends a DELETE request to /items/5 and the service deletes the item successfully?

  @Delete(':id')
  async remove(@Param('id') id: string) {
    await this.itemsService.delete(id);
    return { message: `Item ${id} deleted` };
  }

medium

A. HTTP 404 Not Found

B. { "message": "Item 5 deleted" }

C. { "error": "Item not found" }

D. Empty response with HTTP 204 No Content

Solution

1. Step 1: Analyze the controller method's return value

   The method returns an object with a message string including the id parameter.

2. Step 2: Understand the response for successful deletion

   Since the method returns a JSON object with the message, the client receives that JSON with HTTP 200 OK by default.

3. Final Answer:

   { "message": "Item 5 deleted" } -> Option B

4. Quick Check:

   Return JSON message on success = { "message": "Item 5 deleted" } [OK]

Hint: Returned object becomes JSON response by default in NestJS [OK]

Common Mistakes:

• Expecting HTTP 204 No Content without explicit status
• Assuming error response without checking service
• Confusing DELETE with GET response

4. Identify the error in this NestJS controller method for updating an item:

  @Put(':id')
  async update(@Body() updateDto: UpdateItemDto) {
    return this.itemsService.update(updateDto.id, updateDto);
  }

medium

A. Method should be named create, not update

B. Using @Body instead of @Query for update data

C. UpdateItemDto cannot be used in @Body

D. Missing @Param decorator to get the id from URL

Solution

1. Step 1: Check how the id is obtained

   The method expects id from updateDto.id, but the route has ':id' parameter which is not extracted.

2. Step 2: Identify missing @Param decorator

   To get id from URL, @Param('id') id: string should be added as a parameter to the method.

3. Final Answer:

   Missing @Param decorator to get the id from URL -> Option D

4. Quick Check:

   URL param needs @Param [OK]

Hint: Use @Param to get URL id in update methods [OK]

Common Mistakes:

• Assuming id comes from body without @Param
• Confusing @Body with @Query
• Ignoring route parameters

5. You want to implement a NestJS controller method to update an item by its id from the URL and new data from the request body. Which of the following method signatures correctly follows NestJS CRUD patterns?

hard

A. async update(@Param('id') id: string, @Body() updateDto: UpdateItemDto) { return this.itemsService.update(id, updateDto); }

B. async update(@Body('id') id: string, @Body() updateDto: UpdateItemDto) { return this.itemsService.update(id, updateDto); }

C. async update(@Query('id') id: string, @Body() updateDto: UpdateItemDto) { return this.itemsService.update(id, updateDto); }

D. async update(@Param() params: any) { return this.itemsService.update(params.id, params); }

Solution

1. Step 1: Identify correct decorators for id and body

   Use @Param('id') to get id from URL and @Body() to get update data object.

2. Step 2: Check method parameters and service call

   async update(@Param('id') id: string, @Body() updateDto: UpdateItemDto) { return this.itemsService.update(id, updateDto); } correctly separates id and updateDto and calls service with both.

3. Final Answer:

   async update(@Param('id') id: string, @Body() updateDto: UpdateItemDto) { return this.itemsService.update(id, updateDto); } -> Option A

4. Quick Check:

   Use @Param for id, @Body for data [OK]

Hint: Use @Param('id') and @Body() together for update methods [OK]

Common Mistakes:

• Using @Body('id') instead of @Param('id')
• Using @Query for route params
• Passing params object instead of separate arguments