Services

Services are a specific type of Provider that encapsulate the core business logic of your application.

Overview

In the context of NestJS (and software architecture in general), a Service is responsible for retrieving and storing data. It sits between the Controller (which handles HTTP transport) and the Database/Repository layer.

Technically, a “Service” in NestJS is just a class annotated with @Injectable(). Nest doesn’t differentiate between a “Service” and a “Provider” under the hood—a Service is simply the conceptual name given to a Provider that handles business rules.

Key Concepts

• Separation of Concerns: Controllers should not contain business logic. Services should not know about HTTP requests or responses.
• Reusability: Because a Service abstracts business logic, it can be injected and used by multiple Controllers, CLI commands, or Background Jobs.
• Data Access: Services typically inject Repositories (if using an ORM like TypeORM) or Database Connections to perform CRUD operations.

Code Examples

A Typical Business Logic Service

import { Injectable, NotFoundException } from '@nestjs/common';
import { InjectRepository } from '@nestjs/typeorm';
import { Repository } from 'typeorm';
import { User } from './entities/user.entity';
import { CreateUserDto } from './dto/create-user.dto';

@Injectable()
export class UsersService {
  // Inject the database repository
  constructor(
    @InjectRepository(User)
    private usersRepository: Repository<User>,
  ) {}

  async create(createUserDto: CreateUserDto): Promise<User> {
    // 1. Apply business rules (e.g., hash password, check if email exists)
    const user = this.usersRepository.create(createUserDto);
    
    // 2. Persist to database
    return this.usersRepository.save(user);
  }

  async findOne(id: number): Promise<User> {
    const user = await this.usersRepository.findOneBy({ id });
    if (!user) {
      // 3. Services can throw standard HTTP exceptions which Nest handles automatically
      throw new NotFoundException(`User with ID ${id} not found`);
    }
    return user;
  }
}

Best Practices

• Framework Agnostic: Try to keep your Services as framework-agnostic as possible. Avoid passing Express Request or Response objects into service methods. Pass only the strictly necessary DTOs or primitive values.
• Handle Errors Gracefully: Services should throw appropriate exceptions (like NotFoundException or ConflictException) when business rules are violated, rather than returning nulls that the controller has to constantly check.
• Keep them Small: If a Service grows too large (e.g., thousands of lines), it’s a sign it’s handling too many responsibilities. Break it down into smaller, domain-specific services.