Route parameters in NestJS - Mini Project: Build & Apply

Handling Route Parameters in NestJS

📖 Scenario: You are building a simple NestJS API for a bookstore. You want to create a route that can receive a book's ID as a parameter in the URL and return details about that book.

🎯 Goal: Build a NestJS controller with a route that accepts a bookId as a route parameter and returns a message including that bookId.

📋 What You'll Learn

Create a controller named BooksController.

Add a route handler method named getBookById.

Use the @Param decorator to extract the bookId from the route.

Return a string message that includes the bookId.

💡 Why This Matters

🌍 Real World

Route parameters are used in web APIs to identify specific resources, like books, users, or orders, by their unique IDs in the URL.

💼 Career

Understanding route parameters is essential for backend developers building RESTful APIs with frameworks like NestJS.

1

Create the BooksController class

Create a class called BooksController and decorate it with @Controller('books').

NestJS

@Controller('books')
export class BooksController {
  // Your code here
}

Hint

Use @Controller('books') above the class to define the route prefix.

2

Add the getBookById method with route parameter

Inside BooksController, add a method called getBookById decorated with @Get(':bookId') to handle GET requests with a bookId parameter.

NestJS

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

@Controller('books')
export class BooksController {
  // Your code here
}

Hint

Use @Get(':bookId') to define the route with a parameter named bookId.

3

Extract the bookId parameter using @Param decorator

Modify the getBookById method to accept a parameter called bookId using the @Param('bookId') decorator from @nestjs/common.

NestJS

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

@Controller('books')
export class BooksController {
  @Get(':bookId')
  getBookById(@Param('bookId') bookId: string) {
    // Your code here
  }
}

Hint

Import @Param and use it to get the bookId from the URL.

4

Return a message including the bookId

Inside the getBookById method, return a string like `Book ID requested is: ${bookId}` that includes the bookId parameter.

NestJS

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

@Controller('books')
export class BooksController {
  @Get(':bookId')
  getBookById(@Param('bookId') bookId: string) {
    // Your code here
  }
}

Hint

Use a template string to include the bookId in the returned message.

Practice

1. What does the : symbol represent in a NestJS route path like @Get('users/:id')?

easy

A. It is used to comment out the route path.

B. It defines a query parameter to be passed in the URL.

C. It indicates a static part of the URL path.

D. It marks id as a dynamic route parameter to capture from the URL.

Solution

1. Step 1: Understand route path syntax

   The colon : before id in @Get('users/:id') means id is a variable part of the URL.

2. Step 2: Identify the role of dynamic parameters

   This dynamic parameter allows the route to accept different id values from the URL, making the route flexible.

3. Final Answer:

   It marks id as a dynamic route parameter to capture from the URL. -> Option D

4. Quick Check:

   Colon means dynamic parameter = A [OK]

Hint: Colon before name means dynamic URL part [OK]

Common Mistakes:

• Thinking colon means query parameter
• Confusing static and dynamic parts
• Assuming colon is for comments

2. Which of the following is the correct way to access a route parameter named userId inside a NestJS controller method?

easy

A. @Body('userId') userId: string

B. @Query('userId') userId: string

C. @Param('userId') userId: string

D. @Headers('userId') userId: string

Solution

1. Step 1: Identify decorator for route parameters

   In NestJS, @Param() decorator is used to extract route parameters from the URL.

2. Step 2: Match parameter name

   Using @Param('userId') correctly accesses the userId parameter from the route.

3. Final Answer:

   @Param('userId') userId: string -> Option C

4. Quick Check:

   @Param for route params = C [OK]

Hint: Use @Param('name') to get route parameters [OK]

Common Mistakes:

• Using @Query instead of @Param for route params
• Trying to get route params from @Body or @Headers
• Forgetting to specify parameter name in @Param

3. Given this NestJS controller method:

@Get('products/:id')
getProduct(@Param('id') id: string) {
  return `Product ID is ${id}`;
}

What will be the response when a client requests /products/42?

medium

A. "Product ID is 42"

B. "Product ID is id"

C. "Product ID is undefined"

D. 404 Not Found error

Solution

1. Step 1: Understand route parameter extraction

   The @Param('id') decorator extracts the value 42 from the URL /products/42.

2. Step 2: Check returned string

   The method returns the string with the extracted id interpolated: "Product ID is 42".

3. Final Answer:

   "Product ID is 42" -> Option A

4. Quick Check:

   Route param 'id' = 42 in output [OK]

Hint: Route param value replaces :id in URL [OK]

Common Mistakes:

• Expecting literal 'id' instead of value
• Not using @Param to get the value
• Assuming undefined if param missing

4. What is wrong with this NestJS controller method?

@Get('orders/:orderId')
getOrder(@Param() orderId: string) {
  return `Order: ${orderId}`;
}

medium

A. Missing parameter name inside @Param() decorator.

B. Route path should not have colon before orderId.

C. Method should use @Query instead of @Param.

D. Return statement syntax is invalid.

Solution

1. Step 1: Check @Param usage

   The @Param() decorator without an argument returns an object of all params, not a single string.

2. Step 2: Identify mismatch in parameter type

   The method expects orderId as a string, but @Param() without argument returns an object, causing a type mismatch.

3. Final Answer:

   Missing parameter name inside @Param() decorator. -> Option A

4. Quick Check:

   @Param needs name for single param = B [OK]

Hint: Use @Param('name') for single param, else get object [OK]

Common Mistakes:

• Omitting parameter name in @Param() when expecting string
• Confusing @Param with @Query
• Thinking colon is wrong in route path

5. You want to create a NestJS route that accepts two parameters: category and itemId. Which of the following route paths and method signatures correctly captures both parameters?

hard

A. @Get('shop/:category/:itemId')getItem(@Body('category') category: string, @Body('itemId') itemId: string)

B. @Get('shop/:category/:itemId')getItem(@Param('category') category: string, @Param('itemId') itemId: string)

C. @Get('shop/:category/:itemId')getItem(@Query('category') category: string, @Query('itemId') itemId: string)

D. @Get('shop/:category/:itemId')getItem(@Param() params: string)

Solution

1. Step 1: Verify route path with two parameters

   The path 'shop/:category/:itemId' correctly defines two dynamic route parameters.

2. Step 2: Check method parameters for correct decorators

   Using @Param('category') and @Param('itemId') extracts each parameter individually as strings.

3. Final Answer:

   @Get('shop/:category/:itemId') with getItem(@Param('category') category: string, @Param('itemId') itemId: string) -> Option B

4. Quick Check:

   Use @Param('name') for each route param = D [OK]

Hint: Use @Param('name') for each route parameter [OK]

Common Mistakes:

• Using @Param() with string instead of object
• Using @Query or @Body instead of @Param for route params
• Not matching parameter names exactly