Module augmentation extends existing types without modifying source code. Here's how to use it.
Basic Module Augmentation
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Global Augmentation
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Declaration Merging
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Extending Third-Party Libraries
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Environment Variables
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Extending Built-in Types
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Library Type Definitions
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Path Aliases
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Testing Augmentation
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Best Practices
Organization:
✓ Put declarations in types/ directory
✓ Name files descriptively (express.d.ts)
✓ Include in tsconfig.json
✓ Export {} to make modules
Safety:
✓ Don't override existing types carelessly
✓ Use optional properties when uncertain
✓ Document augmentations
✓ Keep augmentations minimal
Patterns:
✓ Extend Request/Response for auth
✓ Type environment variables
✓ Augment testing matchers
✓ Add global utilities sparingly
Conclusion
Module augmentation extends existing types without modifying source code. Use it for adding properties to Express requests, typing environment variables, extending third-party libraries, and creating global utilities. Keep augmentations organized and documented for maintainability.