Declaration files (.d.ts) provide type information for JavaScript libraries. Here's how to write them effectively.
Basic Declaration Structure
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Global Declarations
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Module Augmentation
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Function Overloads
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Class Declarations
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Namespace Declarations
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Conditional Types in Declarations
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Third-Party Library Example
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Publishing Types
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Best Practices
Structure:
✓ Mirror the library's API structure
✓ Use namespaces for grouping
✓ Export all public types
✓ Document with JSDoc comments
Accuracy:
✓ Match actual library behavior
✓ Use strict type checking
✓ Handle edge cases
✓ Test with real usage
Compatibility:
✓ Support multiple TypeScript versions
✓ Avoid bleeding-edge features
✓ Provide ambient and module forms
✓ Include UMD support if needed
Maintenance:
✓ Version alongside library
✓ Include type tests
✓ Document breaking changes
✓ Accept contributions
Conclusion
Declaration files bridge JavaScript libraries to TypeScript. Match the library's structure, use appropriate declaration forms (module, namespace, global), and test thoroughly. For popular libraries, consider contributing to DefinitelyTyped. Good type definitions make libraries much more usable in TypeScript projects.