Controller decorator in NestJS - Step-by-Step Execution

Concept Flow - Controller decorator

Define Controller Class

↓

Apply @Controller Decorator

↓

NestJS Registers Route Prefix

↓

Define Route Handlers inside Class

↓

Handle Incoming HTTP Requests

↓

Execute Handler Method

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Send Response to Client

The @Controller decorator marks a class as a controller, registering it with NestJS to handle HTTP requests for a specific route prefix.

Execution Sample

NestJS

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

@Controller('cats')
export class CatsController {
  @Get()
  findAll() { return 'All cats'; }
}

Defines a controller for the 'cats' route that returns 'All cats' on GET requests.

Execution Table

Step	Action	Evaluation	Result
1	NestJS reads @Controller('cats') decorator	Registers 'cats' as route prefix	CatsController linked to '/cats' route
2	NestJS reads @Get() decorator on findAll()	Registers GET handler for '/cats'	GET requests to '/cats' call findAll()
3	Incoming HTTP GET request to '/cats'	Matches registered route and method	Calls findAll() method
4	findAll() executes	Returns string 'All cats'	Response body set to 'All cats'
5	Response sent to client	Client receives 'All cats'	Request cycle complete

💡 Request handled and response sent; no further action

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
routePrefix	undefined	'cats'	'cats'	'cats'
handlerMethod	undefined	findAll()	findAll()	findAll()
responseBody	undefined	undefined	'All cats'	'All cats'

Key Moments - 3 Insights

Why does the @Controller decorator take a string like 'cats'?

The string defines the route prefix for all handlers in the class, as shown in execution_table step 1 where 'cats' is registered as the route prefix.

How does NestJS know which method handles GET requests?

The @Get() decorator marks the method as a GET handler, registered in step 2 of the execution_table.

What happens if a request comes to a different route?

NestJS matches routes by prefix and method; if no match is found, the request is not handled by this controller, as implied by step 3 where only '/cats' GET requests call findAll().

Visual Quiz - 3 Questions

Look at the execution_table, what is the route prefix registered at step 1?

A 'GET'

B 'findAll'

C 'cats'

D '@Controller'

💡 Hint

Check the 'Evaluation' column in step 1 of the execution_table.

At which step does the findAll() method execute?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look for the step where the method returns 'All cats' in the execution_table.

If the @Controller decorator was changed to @Controller('dogs'), what changes in the variable_tracker?

A handlerMethod changes to 'dogs'

B routePrefix changes to 'dogs'

C responseBody changes to 'dogs'

D No changes

💡 Hint

Refer to the 'routePrefix' variable in the variable_tracker.

Concept Snapshot

@Controller('route') decorator marks a class as a controller.
It sets the route prefix for all handlers inside.
Methods decorated with @Get(), @Post(), etc., handle HTTP requests.
NestJS uses these to route requests to the right method.
Handlers return responses sent back to clients.

Full Transcript

The Controller decorator in NestJS marks a class as a controller that handles HTTP requests. When you add @Controller('cats'), NestJS registers the class to handle routes starting with '/cats'. Inside the class, methods decorated with @Get(), @Post(), and others define handlers for specific HTTP methods. For example, @Get() on findAll() means GET requests to '/cats' call findAll(). When a request comes in, NestJS matches the route and method, calls the handler, and sends back the response. This flow helps organize your server code by grouping related routes and handlers in classes.