What are controllers in NestJS?

Question

Accepted Answer

**Controllers** handle incoming **HTTP requests** and return responses. They define your application's routes and act as the entry point — receiving requests, extracting data, delegating to services, and returning results. They keep routing concerns separate from business logic.

## Defining a controller

```ts
import { Controller, Get, Post, Body, Param } from "@nestjs/common";

@Controller("users")          // the route PREFIX → all routes start with /users
export class UsersController {
  constructor(private usersService: UsersService) {}   // inject a service via DI

@Get()                       // GET /users
  findAll() {
    return this.usersService.findAll();
  }

@Get(":id")                  // GET /users/:id
  findOne(@Param("id") id: string) {        // extract the route param
    return this.usersService.findOne(id);
  }

@Post()                      // POST /users
  create(@Body() dto: CreateUserDto) {       // extract the request body
    return this.usersService.create(dto);
  }
}
```

## How it works: decorators map methods to routes

```text
@Controller("users") → route prefix /users
@Get() / @Post() / @Put() / @Patch() / @Delete() → HTTP method + sub-path
Combined: @Get(":id") inside @Controller("users") → GET /users/:id
```

Decorators declaratively bind class methods to HTTP routes — clean and readable, no manual route registration.

## Parameter decorators extract request data

```ts
@Get(":id")
example(
  @Param("id") id: string,        // route parameter (/users/:id)
  @Query("sort") sort: string,    // query string (?sort=name)
  @Body() body: CreateUserDto,    // request body
  @Headers("authorization") auth: string,  // a header
  @Req() req: Request,            // the raw request object (rarely needed)
) {}
```

Nest's parameter decorators (`@Param`, `@Query`, `@Body`, `@Headers`) cleanly extract exactly the data you need — no manual parsing of `req`.

## Controllers should be THIN

```ts
// ✅ controller delegates to a service — stays thin
@Post()
create(@Body() dto: CreateUserDto) {
  return this.usersService.create(dto);   // business logic lives in the SERVICE
}
// ❌ don't put database queries / business logic directly in the controller
```

The best practice: keep controllers **thin** — they handle HTTP concerns (routing, extracting data, returning responses) and delegate all business logic to **services**. This separation keeps code testable and organized.

## Why it matters

Controllers are a fundamental NestJS building block — the entry point for all HTTP requests and where your API's routes are defined.

Understanding how decorators (`@Controller`, `@Get`/`@Post`, `@Param`/`@Body`/`@Query`) map methods to routes and extract request data declaratively is essential everyday NestJS knowledge — it's how you build any endpoint.

Equally important is the principle of keeping controllers **thin** (handling only HTTP concerns and delegating business logic to injected services), which embodies NestJS's separation-of-concerns philosophy and keeps code maintainable and testable.

Controllers, together with services and modules, form the core structure of every NestJS application, making them essential to grasp from the start.