Middleware is central to Node.js web frameworks. Here's how to write effective middleware.
Express Middleware Basics
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Common Middleware Types
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Async Middleware
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Error Handling Middleware
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Rate Limiting
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Caching Middleware
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Request Context
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Middleware Composition
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Koa-style Middleware
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Testing Middleware
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Best Practices
Design:
✓ Single responsibility
✓ Configurable via options
✓ Return middleware function
✓ Handle errors properly
Order:
✓ Security first (helmet, cors)
✓ Parsing (json, urlencoded)
✓ Logging
✓ Authentication
✓ Routes
✓ 404 handler
✓ Error handler
Performance:
✓ Early returns when possible
✓ Avoid blocking operations
✓ Use async properly
✓ Cache when appropriate
Testing:
✓ Test in isolation
✓ Test error cases
✓ Test middleware order
✓ Mock dependencies
Conclusion
Middleware is the backbone of Node.js web applications. Write focused, composable middleware functions. Handle errors properly with dedicated error middleware. Use async handlers for Promise-based code. Test middleware in isolation and consider the order carefully.