Controller decorator in NestJS - Mini Project: Build & Apply

Building a Simple NestJS Controller with @Controller Decorator

📖 Scenario: You are creating a simple web server using NestJS. You want to organize your routes by creating a controller that handles HTTP requests for a specific path.

🎯 Goal: Build a NestJS controller class using the @Controller decorator to define a route prefix.

📋 What You'll Learn

Create a controller class named AppController

Use the @Controller decorator with the path 'app'

Add a method named getHello inside the controller

Use the @Get decorator on the getHello method with no path argument

Return the string 'Hello World!' from the getHello method

💡 Why This Matters

🌍 Real World

Controllers in NestJS organize routes and handle HTTP requests in web applications and APIs.

💼 Career

Understanding controllers and decorators is essential for backend development with NestJS, a popular Node.js framework.

1

Create the controller class

Create a class named AppController in your file.

NestJS

# Create a class named AppController
# Your code here

Hint

Use the class keyword followed by AppController and curly braces.

2

Add the @Controller decorator with path 'app'

Add the @Controller('app') decorator above the AppController class definition. Also import Controller from @nestjs/common.

NestJS

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

// Add @Controller('app') decorator below
export class AppController {
}
# Your code here

Hint

Remember to import Controller and place @Controller('app') right before the class.

3

Add a getHello method with @Get decorator

Import Get from @nestjs/common. Inside AppController, add a method named getHello decorated with @Get() that returns the string 'Hello World!'.

NestJS

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

@Controller('app')
export class AppController {
  // Add getHello method with @Get decorator
  // Your code here
}

Hint

Use @Get() above the method and return the exact string 'Hello World!'.

4

Complete the controller export

Ensure the AppController class is exported as default export by adding export default AppController; at the end of the file.

NestJS

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

@Controller('app')
export class AppController {
  @Get()
  getHello(): string {
    return 'Hello World!';
  }
}

// Export AppController as default
# Your code here

Hint

Use export default AppController; at the end of the file to complete the export.

Practice

1. What is the main purpose of the @Controller decorator in NestJS?

easy

A. To define a service that handles business logic

B. To mark a class as a group of routes with a base path

C. To create a database connection

D. To style the HTML output of a component

Solution

1. Step 1: Understand the role of @Controller

   The @Controller decorator marks a class as a handler for HTTP requests grouped under a base path.

2. Step 2: Differentiate from other decorators

   It is not used for services, database, or styling but specifically for routing.

3. Final Answer:

   To mark a class as a group of routes with a base path -> Option B

4. Quick Check:

   @Controller = route group [OK]

Hint: Remember: @Controller groups routes under one path [OK]

Common Mistakes:

• Confusing @Controller with @Injectable
• Thinking it styles components
• Assuming it manages database connections

2. Which of the following is the correct syntax to define a controller with base path 'users' in NestJS?

easy

A. @Controller = 'users'
export class UsersController {}

B. @Controller.users
export class UsersController {}

C. @Controller('users')
export class UsersController {}

D. @Controller path='users'
export class UsersController {}

Solution

1. Step 1: Recall correct decorator syntax

   The @Controller decorator takes the base path as a string argument inside parentheses.

2. Step 2: Check each option's syntax

   Only @Controller('users') export class UsersController {} uses the correct syntax: @Controller('users'). Others misuse dot, equal sign, or invalid syntax.

3. Final Answer:

   @Controller('users') export class UsersController {} -> Option C

4. Quick Check:

   @Controller('path') syntax = @Controller('users') export class UsersController {} [OK]

Hint: Use parentheses with string path in @Controller [OK]

Common Mistakes:

• Using dot notation instead of parentheses
• Assigning with equal sign instead of decorator call
• Writing invalid decorator syntax

3. Given the following controller code, what is the full URL path to access the getAllUsers method?

@Controller('api')
export class UserController {
  @Get('users')
  getAllUsers() {
    return 'List of users';
  }
}

medium

A. /users

B. /users/api

C. /api/getAllUsers

D. /api/users

Solution

1. Step 1: Identify base path from @Controller

   The base path is 'api' from @Controller('api').

2. Step 2: Identify route path from @Get decorator

   The method route is 'users' from @Get('users').

3. Step 3: Combine base and method paths

   Full path is base + method = '/api/users'.

4. Final Answer:

   /api/users -> Option D

5. Quick Check:

   Base + method path = /api/users [OK]

Hint: Combine @Controller and @Get paths for full URL [OK]

Common Mistakes:

• Reversing base and method paths
• Using method name as URL path
• Ignoring base path in URL

4. Identify the error in this NestJS controller code:

@Controller('products')
export class ProductController {
  @Get()
  getAll() {
    return 'All products';
  }

  @Get('')
  getEmpty() {
    return 'Empty route';
  }
}

medium

A. Duplicate routes for the same path cause a conflict

B. Missing parentheses in @Get decorator

C. Controller path should be '/products' with a slash

D. Methods must return JSON objects, not strings

Solution

1. Step 1: Analyze route paths from @Get decorators

   Both methods respond to the same route: base 'products' + '' or no path means '/products'.

2. Step 2: Understand route conflict

   Having two methods for the same route causes a conflict and runtime error.

3. Final Answer:

   Duplicate routes for the same path cause a conflict -> Option A

4. Quick Check:

   Duplicate routes = conflict error [OK]

Hint: Avoid duplicate routes with same path in one controller [OK]

Common Mistakes:

• Focusing on @Get parentheses instead of duplicate routes
• Believing controller path needs leading slash
• Assuming string return type is invalid

5. You want to create a controller that handles routes for both '/admin' and '/user' paths using a single class. Which approach correctly uses the @Controller decorator to achieve this?

hard

A. Create two separate controllers, one for 'admin' and one for 'user'

B. @Controller(['admin', 'user'])
export class MultiPathController {}

C. @Controller('admin,user')
export class MultiPathController {}

D. @Controller('admin')
@Controller('user')
export class MultiPathController {}

Solution

1. Step 1: Understand @Controller supports single base path

   The @Controller decorator accepts a single string as the base path. It does not support multiple paths in an array.

2. Step 2: Evaluate options for multiple paths

   Create two separate controllers, one for 'admin' and one for 'user' is correct because creating two separate controllers is the proper way to handle multiple base paths. @Controller(['admin', 'user']) export class MultiPathController {} is invalid syntax. @Controller('admin,user') export class MultiPathController {} uses a comma-separated string which is treated as a single path. @Controller('admin') @Controller('user') export class MultiPathController {} uses multiple decorators which is invalid.

3. Final Answer:

   Create two separate controllers, one for 'admin' and one for 'user' -> Option A

4. Quick Check:

   One controller per base path [OK]

Hint: Use separate controllers for different base paths [OK]

Common Mistakes:

• Using array of strings in @Controller (not supported)
• Applying multiple @Controller decorators on one class
• Trying to combine multiple paths in one string