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Guidelines for building robust APIs with Express.js and TypeScript, covering middleware patterns, routing, and security best practices
Express TypeScript Development
You are an expert in Express.js and TypeScript development with deep knowledge of building scalable, maintainable APIs.
TypeScript General Guidelines
Basic Principles
Use English for all code and documentation
Always declare types for variables and functions (parameters and return values)
Avoid using any type - create necessary types instead
Use JSDoc to document public classes and methods
Write concise, maintainable, and technically accurate code
Use functional and declarative programming patterns; avoid classes where possible
Prefer iteration and modularization to adhere to DRY principles
Nomenclature
Use PascalCase for types and interfaces
Use camelCase for variables, functions, and methods
Use kebab-case for file and directory names
Use UPPERCASE for environment variables
Use descriptive variable names with auxiliary verbs: isLoading, hasError, canDelete
Start each function with a verb
Functions
Write short functions with a single purpose
Use arrow functions for middleware and handlers
Use async/await consistently throughout the codebase
Use the RO-RO pattern for multiple parameters
Types and Interfaces
Prefer interfaces over types for object shapes
Avoid enums; use maps or const objects instead
Use Zod for runtime validation with inferred types
Use readonly for immutable properties
Express-Specific Guidelines
Project Structure
src/
routes/
{resource}/
index.ts
controller.ts
validators.ts
middleware/
auth.ts
errorHandler.ts
requestLogger.ts
validateRequest.ts
services/
{domain}Service.ts
models/
{entity}.ts
types/
express.d.ts
index.ts
utils/
config/
app.ts
server.ts
Application Setup
import express, { Express } from 'express';
import helmet from 'helmet';
import cors from 'cors';
import { errorHandler } from './middleware/errorHandler';
import { requestLogger } from './middleware/requestLogger';
import routes from './routes';
const createApp = (): Express => {
const app = express();
// Security middleware
app.use(helmet());
app.use(cors());
// Body parsing
app.use(express.json());
app.use(express.urlencoded({ extended: true }));
// Request logging
app.use(requestLogger);
// Routes
app.use('/api', routes);
// Error handling (must be last)
app.use(errorHandler);
return app;
};
export default createApp;
Middleware Patterns
Use middleware for cross-cutting concerns
Chain middleware in order of execution
Handle errors in dedicated error middleware
import { Request, Response, NextFunction } from 'express';
// Request logging middleware
const requestLogger = (req: Request, res: Response, next: NextFunction): void => {
const start = Date.now();
res.on('finish', () => {
const duration = Date.now() - start;
console.log(`${req.method} ${req.path} ${res.statusCode} ${duration}ms`);
});
next();
};
// Authentication middleware
const authenticate = async (req: Request, res: Response, next: NextFunction): Promise<void> => {
try {
const token = req.headers.authorization?.split(' ')[1];
if (!token) {
res.status(401).json({ error: 'No token provided' });
return;
}
const user = await verifyToken(token);
req.user = user;
next();
} catch (error) {
res.status(401).json({ error: 'Invalid token' });
}
};
Routing
Organize routes by resource
Use Router for modular route definitions
Apply middleware at appropriate levels
import { Router } from 'express';
import { authenticate } from '../middleware/auth';
import { validateRequest } from '../middleware/validateRequest';
import { createUserSchema, updateUserSchema } from './validators';
import * as controller from './controller';
const router = Router();
router.get('/', controller.listUsers);
router.get('/:id', controller.getUser);
router.post('/', validateRequest(createUserSchema), controller.createUser);
router.put('/:id', authenticate, validateRequest(updateUserSchema), controller.updateUser);
router.delete('/:id', authenticate, controller.deleteUser);
export default router;
Request Validation
Validate all incoming requests
Use Zod for schema definition and validation
Create reusable validation middleware
import { z } from 'zod';
import { Request, Response, NextFunction } from 'express';
const createUserSchema = z.object({
body: z.object({
name: z.string().min(1),
email: z.string().email(),
password: z.string().min(8),
}),
});
const validateRequest = (schema: z.ZodSchema) => {
return async (req: Request, res: Response, next: NextFunction): Promise<void> => {
try {
await schema.parseAsync({
body: req.body,
query: req.query,
params: req.params,
});
next();
} catch (error) {
if (error instanceof z.ZodError) {
res.status(400).json({
error: 'Validation failed',
details: error.errors,
});
return;
}
next(error);
}
};
};
Error Handling
Create custom error classes
Use centralized error handler middleware
Return consistent error responses
class AppError extends Error {
constructor(
public statusCode: number,
public message: string,
public code: string = 'INTERNAL_ERROR'
) {
super(message);
this.name = 'AppError';
}
}
class NotFoundError extends AppError {
constructor(resource: string) {
super(404, `${resource} not found`, 'NOT_FOUND');
}
}
// Error handler middleware
const errorHandler = (err: Error, req: Request, res: Response, next: NextFunction): void => {
if (err instanceof AppError) {
res.status(err.statusCode).json({
error: err.code,
message: err.message,
});
return;
}
console.error(err);
res.status(500).json({
error: 'INTERNAL_ERROR',
message: 'An unexpected error occurred',
});
};
TypeScript Extensions
Extend Express types for custom properties:
// types/express.d.ts
import { User } from '../models/User';
declare global {
namespace Express {
interface Request {
user?: User;
requestId?: string;
}
}
}
Security Best Practices
Use helmet for security headers
Implement rate limiting
Sanitize user inputs
Use HTTPS in production
Implement CORS properly
import helmet from 'helmet';
import rateLimit from 'express-rate-limit';
app.use(helmet());
const limiter = rateLimit({
windowMs: 15 * 60 * 1000, // 15 minutes
max: 100, // limit each IP to 100 requests per windowMs
});
app.use('/api', limiter);
Testing
Use Jest with supertest for integration tests
Test middleware in isolation
Mock external dependencies
import request from 'supertest';
import createApp from '../app';
describe('Users API', () => {
const app = createApp();
it('should create a user', async () => {
const response = await request(app)
.post('/api/users')
.send({ name: 'John', email: 'john@example.com', password: 'password123' })
.expect(201);
expect(response.body).toHaveProperty('id');
expect(response.body.name).toBe('John');
});
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