Controller decorator in NestJS

Introduction

The Controller decorator marks a class as a controller that handles incoming requests and sends responses in a NestJS app.

When you want to group related request handlers for a specific part of your app.

When you need to define routes that respond to HTTP requests like GET or POST.

When organizing your backend code into clear sections for different features.

When you want to separate logic for handling requests from other parts like services.

When building REST APIs or web apps that respond to user actions.

Syntax

NestJS

@Controller('route-path')
export class SomeController {
  // handler methods here
}

The string inside @Controller() sets the base route path for all handlers in the class.

If you leave it empty like @Controller(), it will handle requests at the root path.

Examples

This controller handles all routes that start with /users.

NestJS

@Controller('users')
export class UsersController {
  // handles routes starting with /users
}

This controller handles routes at the root path /.

NestJS

@Controller()
export class AppController {
  // handles routes at the root path /
}

Groups all product-related routes under /products.

NestJS

@Controller('products')
export class ProductsController {
  // add methods to handle product routes
}

Sample Program

This example creates a controller with the base route /hello. It has one GET handler that returns a greeting string.

NestJS

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

@Controller('hello')
export class HelloController {
  @Get()
  sayHello() {
    return 'Hello, NestJS!';
  }
}

Important Notes

Controllers should be simple and only handle routing and request/response logic.

Use decorators like @Get(), @Post() inside controllers to define specific routes.

Controllers are automatically discovered if registered in a module.

Summary

@Controller marks a class as a request handler group in NestJS.

It sets a base path for all routes inside the class.

Use it to organize your app's routes clearly and cleanly.

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