Query parameters in NestJS - Mini Project: Build & Apply

Handling Query Parameters in NestJS

📖 Scenario: You are building a simple NestJS API that returns a list of books. You want to allow users to filter books by genre using query parameters.

🎯 Goal: Create a NestJS controller that reads a genre query parameter from the URL and returns only books matching that genre.

📋 What You'll Learn

Create a controller with a GET route /books

Use the @Query() decorator to read the genre query parameter

Filter the list of books by the genre parameter if it is provided

Return the filtered list of books as JSON

💡 Why This Matters

🌍 Real World

APIs often need to filter data based on user input in the URL. Query parameters let users specify filters without changing the route.

💼 Career

Backend developers use query parameters to build flexible APIs that serve filtered data, improving user experience and performance.

1

Set up the books data array

Create a variable called books that is an array of objects. Each object should have title and genre properties with these exact entries: { title: 'The Hobbit', genre: 'Fantasy' }, { title: '1984', genre: 'Dystopian' }, { title: 'The Catcher in the Rye', genre: 'Classic' }.

NestJS

# Create the books array with the given objects
# Your code here

Need a hint?

Use const books = [ ... ] to create the array with the exact objects.

2

Create the controller and import decorators

Create a NestJS controller class called BooksController. Import Controller and Get from @nestjs/common. Add the @Controller('books') decorator above the class.

NestJS

const books = [
  { title: 'The Hobbit', genre: 'Fantasy' },
  { title: '1984', genre: 'Dystopian' },
  { title: 'The Catcher in the Rye', genre: 'Classic' }
];

// Import Controller and Get from @nestjs/common
// Create BooksController class with @Controller('books') decorator
# Your code here

Need a hint?

Use @Controller('books') above the class and import the decorators from @nestjs/common.

3

Add a GET method with query parameter

Inside BooksController, add a method called getBooks with the @Get() decorator. Use the @Query('genre') decorator to get the genre query parameter as a string argument. Filter the books array to only include books where book.genre === genre if genre is provided. Return the filtered array or all books if no genre is given.

NestJS

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

const books = [
  { title: 'The Hobbit', genre: 'Fantasy' },
  { title: '1984', genre: 'Dystopian' },
  { title: 'The Catcher in the Rye', genre: 'Classic' }
];

@Controller('books')
export class BooksController {
  // Add getBooks method with @Get() decorator
  // Use @Query('genre') to get genre parameter
  // Filter books by genre if provided
  // Return filtered or all books
  # Your code here
}

Need a hint?

Import @Query and use it to read the genre query parameter. Use filter to select matching books.

4

Export the controller and finalize

Make sure the BooksController class is exported with export class BooksController. This completes the controller setup to handle query parameters.

NestJS

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

const books = [
  { title: 'The Hobbit', genre: 'Fantasy' },
  { title: '1984', genre: 'Dystopian' },
  { title: 'The Catcher in the Rye', genre: 'Classic' }
];

@Controller('books')
// Ensure BooksController is exported
# Your code here

Need a hint?

Use export class BooksController to make the controller available to the app.