At first, it is common to place logic directly inside a controller. This works for simple cases, but quickly becomes hard to manage as the application grows.

Here is an example of logic inside a controller:

@Controller('users')
export class UsersController {
  @Get()
  getUsers() {
    return ['Alice', 'Bob'];
  }
}

This approach mixes request handling and business logic in one place.

To improve this, move the logic into a service.

Create a service:

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

@Injectable()
export class UsersService {
  getUsers() {
    return ['Alice', 'Bob'];
  }
}

Now update the controller:

import { Controller, Get } from '@nestjs/common';
import { UsersService } from './users.service';

@Controller('users')
export class UsersController {
  constructor(private usersService: UsersService) {}

  @Get()
  getUsers() {
    return this.usersService.getUsers();
  }
}

Here is what changed:

• The controller no longer contains data or logic;
• The service handles all processing;
• The controller only calls the service and returns the result.

This separation makes your code easier to read, test, and extend.