Understanding when to use interfaces versus types is essential for TypeScript development. Here's a comprehensive comparison.
Basic Syntax
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Declaration Merging
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Extension and Intersection
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Unique Type Features
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Unique Interface Features
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Performance Considerations
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Practical Guidelines
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Common Patterns
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Combining Both
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Migration Considerations
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Best Practices
Default Choice:
✓ Use interface for object types
✓ Use type for unions, tuples, mapped types
✓ Be consistent within a codebase
✓ Document team conventions
Interface When:
✓ Defining object shapes
✓ Creating class contracts
✓ Building extensible APIs
✓ You need declaration merging
Type When:
✓ Creating union types
✓ Creating tuple types
✓ Creating mapped/conditional types
✓ You need type computations
Avoid:
✗ Mixing styles inconsistently
✗ Over-engineering type definitions
✗ Complex deeply nested types
✗ Circular type references
Conclusion
Both interfaces and types are powerful TypeScript features. Use interfaces for object shapes and class contracts where declaration merging might be useful. Use types for unions, tuples, and complex type computations. In practice, the choice often comes down to team conventions and specific use cases.