CRUD operations in NestJS - Mini Project: Build & Apply

CRUD Operations with NestJS

📖 Scenario: You are building a simple NestJS backend to manage a list of books in a library. Each book has an id, title, and author.This backend will allow users to create, read, update, and delete books.

🎯 Goal: Build a NestJS service that performs basic CRUD operations on an in-memory list of books.

📋 What You'll Learn

Create an initial array of books with exact entries

Add a variable to track the next book ID

Implement methods to create, read, update, and delete books

Export the service class with all CRUD methods

💡 Why This Matters

🌍 Real World

Backend services often need to manage data with create, read, update, and delete operations. This project shows how to do that simply with NestJS.

💼 Career

Understanding CRUD operations in NestJS is essential for backend developers building APIs and services.

1

Data Setup: Create initial books array

Create a variable called books that is an array containing these exact objects: { id: 1, title: '1984', author: 'George Orwell' } and { id: 2, title: 'Brave New World', author: 'Aldous Huxley' }.

NestJS

// Create the books array with two book objects
// Your code here

Hint

Use const books = [ ... ] with two objects inside.

2

Configuration: Add nextId variable

Add a variable called nextId and set it to 3 to track the next book ID.

NestJS

const books = [
  { id: 1, title: '1984', author: 'George Orwell' },
  { id: 2, title: 'Brave New World', author: 'Aldous Huxley' }
];

// Add nextId variable here
// Your code here

Hint

Use let nextId = 3; to allow incrementing later.

3

Core Logic: Implement CRUD methods in BooksService

Create a class called BooksService with these methods: findAll() that returns books, findOne(id) that returns the book with matching id, create(title, author) that adds a new book with a new id, update(id, title, author) that updates the matching book, and remove(id) that deletes the book with the given id. Use the variables books and nextId inside the class.

NestJS

const books = [
  { id: 1, title: '1984', author: 'George Orwell' },
  { id: 2, title: 'Brave New World', author: 'Aldous Huxley' }
];

let nextId = 3;

// Create BooksService class with CRUD methods
// Your code here

Hint

Define a class with methods using the books array and nextId variable.

4

Completion: Export the BooksService class

Add export default BooksService; at the end of the file to export the service class.

NestJS

const books = [
  { id: 1, title: '1984', author: 'George Orwell' },
  { id: 2, title: 'Brave New World', author: 'Aldous Huxley' }
];

let nextId = 3;

class BooksService {
  findAll() {
    return books;
  }

  findOne(id) {
    return books.find(book => book.id === id);
  }

  create(title, author) {
    const newBook = { id: nextId++, title, author };
    books.push(newBook);
    return newBook;
  }

  update(id, title, author) {
    const book = books.find(book => book.id === id);
    if (book) {
      book.title = title;
      book.author = author;
    }
    return book;
  }

  remove(id) {
    const index = books.findIndex(book => book.id === id);
    if (index !== -1) {
      books.splice(index, 1);
      return true;
    }
    return false;
  }
}

// Export the BooksService class
// Your code here

Hint

Use export default BooksService; at the end.

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