Express Typescript API Best Practices

Express + TypeScript REST API Best Practices

Professional skill for building production-ready REST APIs with Express.js and TypeScript, following SOLID principles and enterprise-grade architectural patterns.

Core Principles

1. Layered Architecture (Separation of Concerns)

Follow strict layer separation to achieve Single Responsibility Principle (SRP) and Dependency Inversion Principle (DIP):

Routes → Controllers → Services → Models → Database

Layer Responsibilities:

• Routes (src/v1/routes/): Define HTTP endpoints, apply middleware
• Controllers (src/controllers/): Handle HTTP request/response, format data
• Services (src/services/): Contain business logic, orchestrate transactions
• Models (src/models/): Define data structures (ORM models)
• Middlewares (src/middlewares/): Cross-cutting concerns (auth, validation)

Implementation Rules:

// ❌ BAD: Business logic in controller
export const createUser = async (req: Request, res: Response) => {
  const hashedPassword = await argon2.hash(req.body.password);
  const user = await User.create({ ...req.body, password: hashedPassword });
  res.json(user);
};

// ✅ GOOD: Controller delegates to service
export const createUser = async (req: Request, res: Response) => {
  try {
    const user = await crearUsuario(req.body);
    res.status(201).json({
      status: "ok",
      message: "Usuario creado correctamente",
      data: user
    });
  } catch (error: any) {
    res.status(500).json({
      status: "error",
      message: "Error al crear usuario",
      error: error.message
    });
  }
};

2. Standardized API Response Format

ALL endpoints MUST return consistent response structure:

// Success response
{
  status: "ok",
  message: string,
  data?: any  // Optional
}

// Error response
{
  status: "error",
  message: string,
  error?: string,     // Detailed error
  code?: string,      // Error code (e.g., "TOKEN_INVALID")
  detalles?: object   // Additional context
}

HTTP Status Codes:

• 200 - Success (GET, PUT, DELETE)
• 201 - Resource created (POST)
• 400 - Validation error
• 401 - Unauthorized (missing/invalid token)
• 403 - Forbidden (insufficient permissions)
• 404 - Resource not found
• 500 - Server error

3. Transaction Management Pattern

For operations involving multiple database changes, ALWAYS use transactions:

export const crearDetalleVenta = async (detalleVentaData: any[]) => {
  const transaction = await sequelize.transaction();

  try {
    // 1. Validate input
    if (!Array.isArray(detalleVentaData) || detalleVentaData.length === 0) {
      throw new Error('detalleVentaData debe ser un array no vacío');
    }

    // 2. Batch load related data (optimization)
    const productoIds = [...new Set(detalleVentaData.map(i => i.producto_id))];
    const productos = await Producto.findAll({
      where: { id: productoIds },
      transaction
    });
    const productoMap = new Map(productos.map(p => [p.id, p]));

    // 3. Process each item with business rules
    for (const item of detalleVentaData) {
      const producto = productoMap.get(item.producto_id);
      if (!producto) throw new Error(`Producto ${item.producto_id} no encontrado`);

      // Auto-fill fields
      item.precio_unitario = item.precio_unitario ?? Number(producto.precio_minorista);
      item.sub_total = item.sub_total ?? parseFloat((item.precio_unitario * item.cantidad).toFixed(2));

      // Create related records (stock movement)
      const movimiento = await crearMovimiento({
        tipo: "salida",
        producto_id: item.producto_id,
        almacen_id: item.almacen_id,
        cantidad: item.cantidad,
        precio_unitario: item.precio_unitario,
        descripcion: `Salida por venta ID: ${item.venta_id}`
      }, transaction);

      item.movimiento_id = movimiento.id;
    }

    // 4. Bulk insert (performance)
    const detalles = await DetalleVenta.bulkCreate(detalleVentaData, { transaction });

    // 5. Update parent record with locking
    const venta = await Venta.findOne({
      where: { id: detalleVentaData[0].venta_id },
      transaction,
      lock: transaction.LOCK.UPDATE  // Prevent race conditions
    });

    if (!venta) throw new Error('Venta no encontrada');

    const totalSubtotales = detalleVentaData.reduce((sum, d) => sum + d.sub_total, 0);
    if (venta.total === 0 || venta.total < totalSubtotales) {
      venta.total = totalSubtotales;
      await venta.save({ transaction });
    }

    await transaction.commit();
    return detalles;
  } catch (error) {
    await transaction.rollback();
    console.error('Error en transacción:', error);
    throw error;
  }
};

Transaction Best Practices:

• Pass transaction to ALL database operations within the scope
• Use optimistic locking (lock: transaction.LOCK.UPDATE) for concurrent updates
• Batch load related data BEFORE the loop (N+1 query prevention)
• Always rollback on error, commit on success
• Validate ALL input at the beginning

4. JWT Authentication & Role-Based Authorization (RBAC)

Authentication Middleware (autenticarToken):

export const autenticarToken = (req: AuthRequest, res: Response, next: NextFunction): void => {
  const authHeader = req.headers['authorization'];
  const token = authHeader && authHeader.split(' ')[1]; // Bearer TOKEN

  if (!token) {
    res.status(401).json({
      status: "error",
      message: "Acceso no autorizado",
      error: "No se proporcionó un token de autenticación",
      code: "TOKEN_NOT_PROVIDED"
    });
    return;
  }

  try {
    const payload = verificarToken(token);
    req.usuario = payload;  // Attach user to request
    next();
  } catch (error) {
    res.status(403).json({
      status: "error",
      message: "Token inválido",
      error: "El token ha expirado o es inválido",
      code: "TOKEN_INVALID"
    });
  }
};

Authorization Middleware (verificarRol):

export const verificarRol = (...rolesPermitidos: string[]) => {
  return async (req: AuthRequest, res: Response, next: NextFunction): Promise<void> => {
    if (!req.usuario) {
      res.status(401).json({
        status: "error",
        message: "Autenticación requerida",
        code: "AUTHENTICATION_REQUIRED"
      });
      return;
    }

    // Cache roles in request to avoid multiple DB queries
    if (!req.usuario.roles) {
      const rolesAsignados = await UsuarioRol.findAll({
        where: { usuario_id: req.usuario.id },
        include: [{ model: Rol, as: "rol" }]
      });
      req.usuario.roles = rolesAsignados.map((ur: any) => ur.rol.nombre);
    }

    const tienePermiso = (req.usuario.roles || []).some(rol =>
      rolesPermitidos.includes(rol)
    );

    if (!tienePermiso) {
      res.status(403).json({
        status: "error",
        message: "Permisos insuficientes",
        error: `Requiere rol: ${formatearRoles(rolesPermitidos)}`,
        code: "INSUFFICIENT_PERMISSIONS",
        detalles: {
          rolesRequeridos: rolesPermitidos,
          usuario: req.usuario.username
        }
      });
      return;
    }

    next();
  };
};

Usage in Routes:

// Apply globally to all routes
app.use("/v1/usuarios", autenticarToken, usuariosRouter);

// Apply per-route with role checking
router.post("/", verificarRol("admin"), createUsuario);
router.get("/", verificarRol("admin", "gerente"), getUsuarios);

Many-to-Many Role System:

• Users can have multiple roles (flexibility)
• Roles cached in request object (performance)
• Database-driven (no hardcoded roles)

5. Input Validation with Zod

Define schemas in src/schemas/:

import { z } from "zod";

export const createUsuarioSchema = z.object({
  body: z.object({
    username: z.string()
      .min(3, "Username debe tener al menos 3 caracteres")
      .max(50, "Username no puede exceder 50 caracteres"),
    email: z.string()
      .email("Email inválido")
      .max(100),
    password: z.string()
      .min(8, "Contraseña debe tener al menos 8 caracteres")
      .max(100),
    rol_id: z.number().int().positive().optional(),
    mustChangePassword: z.boolean().optional()
  }),
  params: z.object({}),
  query: z.object({})
});

export type CreateUsuarioInput = z.infer<typeof createUsuarioSchema>;

Validation Middleware:

const validate = (schema: ZodObject<any, any>) => {
  return async (req: Request, res: Response, next: NextFunction): Promise<void> => {
    try {
      const result = await schema.safeParseAsync({
        body: req.body,
        query: req.query,
        params: req.params
      });

      if (!result.success) {
        const formattedErrors = result.error.issues.map(error => ({
          field: error.path[1] || error.path[0],
          message: error.message
        }));

        res.status(400).json({
          status: "error",
          message: "Error de validación",
          errors: formattedErrors
        });
        return;
      }

      // Replace with validated data
      if (result.data.body) req.body = result.data.body;
      if (result.data.query) req.query = result.data.query as any;
      if (result.data.params) req.params = result.data.params as any;

      next();
    } catch (error) {
      res.status(500).json({
        status: "error",
        message: "Error interno en validación"
      });
    }
  };
};

Usage:

router.post("/", validate(createUsuarioSchema), createUsuario);

6. Dynamic Data Inclusion Pattern

Support flexible data loading via query parameters:

// Controller: Parse query params
function construirInclusions(include: string) {
  return String(include || "")
    .split(",")
    .map(s => s.trim())
    .filter(Boolean);
}

export const getUsuarios = async (req: Request, res: Response) => {
  const includeList = construirInclusions(req.query.include as string);
  const usuarios = await obtenerUsuarios({ include: includeList });
  res.json({ status: "ok", data: usuarios });
};

// Service: Build Sequelize includes
type Opts = {
  include?: string[];
}

const construirInclusions = (opts: Opts = {}): any[] => {
  const include: any[] = [];

  if (opts.include?.includes("roles")) {
    include.push({
      model: Rol,
      as: "roles",
      through: { attributes: [] }  // Hide join table
    });
  }

  if (opts.include?.includes("almacen")) {
    include.push({ model: Almacen, as: "almacen" });
  }

  return include;
};

export const obtenerUsuarios = async (opts: Opts = {}) => {
  const include = construirInclusions(opts);
  return await Usuario.findAll({ include });
};

Usage:

GET /v1/usuarios?include=roles
GET /v1/detalle_ventas?include=producto,almacen

7. OpenAPI/Swagger Documentation

Configure comprehensive API documentation:

// src/config/swagger.ts
const swaggerDefinition = {
  openapi: '3.0.0',
  info: {
    title: 'API Sistema',
    version: '1.0.0',
    description: 'REST API con autenticación JWT y RBAC'
  },
  servers: [
    { url: `http://${SERVER_HOST}:${SERVER_PORT}`, description: 'Desarrollo' }
  ],
  components: {
    securitySchemes: {
      BearerAuth: {
        type: 'http',
        scheme: 'bearer',
        bearerFormat: 'JWT'
      }
    },
    schemas: {
      SuccessResponse: {
        type: 'object',
        properties: {
          status: { type: 'string', enum: ['ok'] },
          message: { type: 'string' },
          data: { type: 'object' }
        }
      },
      ErrorResponse: {
        type: 'object',
        properties: {
          status: { type: 'string', enum: ['error'] },
          message: { type: 'string' },
          error: { type: 'string' },
          code: { type: 'string' }
        }
      }
    }
  },
  paths: {
    '/v1/usuarios': {
      get: {
        tags: ['Usuarios'],
        security: [{ BearerAuth: [] }],
        responses: {
          200: { description: 'Success' },
          401: { $ref: '#/components/responses/UnauthorizedError' }
        }
      }
    }
  }
};

Mount in Express:

import swaggerUi from "swagger-ui-express";
import { swaggerSpec } from "./config/swagger";

app.use("/api-docs", swaggerUi.serve, swaggerUi.setup(swaggerSpec, {
  customSiteTitle: "API Documentation",
  customCss: '.swagger-ui .topbar { display: none }'
}));

8. Security Best Practices

Password Hashing (Argon2):

import argon2 from "argon2";

// Hashing
const hashedPassword = await argon2.hash(password);

// Verification
const isValid = await argon2.verify(hashedPassword, password);

Environment Variables:

// src/config/config.ts
import dotenv from "dotenv";
dotenv.config();

export const SERVER_HOST = process.env.SERVER_HOST || "localhost";
export const SERVER_PORT = Number(process.env.SERVER_PORT) || 3000;
export const DB_HOST = process.env.DB_HOST || "localhost";
export const DB_USER = process.env.DB_USER!;
export const DB_PASS = process.env.DB_PASS!;
export const DB_NAME = process.env.DB_NAME!;

JWT Configuration:

// src/config/jwt.ts
import jwt from "jsonwebtoken";

const SECRET_KEY = process.env.JWT_SECRET || "your-secret-key";

export const generarToken = (payload: any): string => {
  return jwt.sign(payload, SECRET_KEY, { expiresIn: "1h" });
};

export const verificarToken = (token: string): any => {
  return jwt.verify(token, SECRET_KEY);
};

9. Project Structure

src/
├── config/
│   ├── db.ts              # Database connection
│   ├── config.ts          # Environment variables
│   ├── jwt.ts             # JWT utilities
│   └── swagger.ts         # OpenAPI definition
├── models/
│   ├── usuarios/
│   │   ├── usuarios.ts
│   │   ├── roles.ts
│   │   ├── usuarios_roles.ts
│   │   └── associations.ts  # Model relationships
│   ├── productos/
│   └── ventas/
├── controllers/
│   ├── usuarios/
│   │   └── usuarios.controller.ts
│   └── ventas/
├── services/
│   ├── usuarios/
│   │   ├── usuarios.service.ts
│   │   └── roles.service.ts
│   └── ventas/
├── middlewares/
│   ├── auth.middleware.ts     # autenticarToken, verificarRol
│   └── validateResource.ts    # Zod validation
├── schemas/
│   ├── usuarios.schema.ts
│   └── ventas.schema.ts
├── v1/
│   └── routes/
│       ├── usuarios/
│       │   └── usuarios.route.ts
│       └── ventas/
├── scripts/
│   └── seed-admin.ts          # Database seeding
└── index.ts                   # App entry point

10. Database Patterns

Model Associations (Sequelize):

// src/models/usuarios/associations.ts
import Usuario from "./usuarios";
import Rol from "./roles";
import UsuarioRol from "./usuarios_roles";

// Many-to-Many
Usuario.belongsToMany(Rol, {
  through: UsuarioRol,
  foreignKey: "usuario_id",
  otherKey: "rol_id",
  as: "roles"
});

Rol.belongsToMany(Usuario, {
  through: UsuarioRol,
  foreignKey: "rol_id",
  otherKey: "usuario_id",
  as: "usuarios"
});

// One-to-Many for join table
UsuarioRol.belongsTo(Usuario, { foreignKey: "usuario_id", as: "usuario" });
UsuarioRol.belongsTo(Rol, { foreignKey: "rol_id", as: "rol" });

Database Initialization:

// src/index.ts
app.listen(PORT, HOST, async () => {
  try {
    await sequelize.authenticate();
    console.log("Database connected");

    // sync({ force: false }) preserves data
    // sync({ force: true }) drops and recreates (DANGER!)
    await sequelize.sync({ force: false });
    console.log("Models synchronized");
  } catch (error) {
    console.error("Database connection failed:", error);
  }
});

SOLID Principles Application

S - Single Responsibility Principle

• Each layer has ONE responsibility
• Controllers: HTTP handling
• Services: Business logic
• Models: Data structure

O - Open/Closed Principle

• Middleware composition allows extension without modification
• Dynamic includes support new relations without changing core logic

L - Liskov Substitution Principle

• Consistent response format allows interchangeable endpoints
• Middleware can be swapped without breaking the chain

I - Interface Segregation Principle

• Zod schemas define minimal required fields
• Optional include parameter avoids forcing unnecessary data

D - Dependency Inversion Principle

• Controllers depend on service abstractions, not concrete implementations
• Services use ORM models (abstraction) rather than direct SQL

Quick Start Checklist

When creating a new REST API endpoint:

1. ✅ Define Zod schema in src/schemas/
2. ✅ Create service function in src/services/ with business logic
3. ✅ Use transactions for multi-step operations
4. ✅ Create controller in src/controllers/ that calls service
5. ✅ Return standardized response format
6. ✅ Define route in src/v1/routes/
7. ✅ Apply autenticarToken middleware if protected
8. ✅ Apply verificarRol middleware if role-restricted
9. ✅ Apply validate(schema) middleware for input validation
10. ✅ Document in Swagger (src/config/swagger.ts)

Common Patterns

For detailed implementations and code examples, see:

• Architecture Deep Dive - Detailed layer explanations
• Transaction Patterns - Complex transaction scenarios
• Authentication & RBAC - Complete auth implementation
• Validation Strategies - Zod patterns and custom validators
• Error Handling - Comprehensive error management
• Testing Strategies - Unit and integration test patterns
• Performance Optimization - Query optimization, caching

Anti-Patterns to Avoid

❌ DON'T:

• Put business logic in controllers
• Make database calls from controllers
• Forget transactions for multi-step operations
• Hardcode role names in code (use database)
• Return raw error messages to clients
• Skip input validation
• Use sync({ force: true }) in production
• Expose sensitive data in JWT payload
• Use findAll() without pagination (for large datasets)

✅ DO:

• Keep controllers thin (delegate to services)
• Wrap related operations in transactions
• Validate all input with Zod
• Return standardized response format
• Hash passwords with Argon2
• Cache user roles to avoid repeated DB queries
• Use environment variables for configuration
• Document all endpoints in Swagger

Skill Version: 1.0.0 Author: Miller Marru (@MILLERMARRU) Contact: millermarru4@gmail.com Repository: https://github.com/MILLERMARRU/express-typescript-api-best-practices License: MIT Last Updated: 2026-02-05