Type predicates enable custom type narrowing with user-defined type guards. Here's how to use them effectively.
Basic Type Predicates
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Common Type Guards
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Interface Type Guards
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Discriminated Union Guards
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Array Type Guards
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Assertion Functions
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Generic Type Guards
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Complex Type Guards
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Practical Patterns
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Best Practices
Design:
✓ Use type predicates for reusable guards
✓ Combine with generics for flexibility
✓ Validate all required properties
✓ Handle edge cases (null, undefined)
Safety:
✓ Check types thoroughly
✓ Use assertion functions for invariants
✓ Validate at system boundaries
✓ Test guards with edge cases
Performance:
✓ Order checks by likelihood
✓ Fail fast on obvious mismatches
✓ Cache validation results if repeated
✓ Avoid complex recursive checks
Avoid:
✗ Type casting without validation
✗ Incomplete property checks
✗ Silent failures in assertions
✗ Over-complicated guards
Conclusion
Type predicates enable powerful custom type narrowing in TypeScript. Use them to create reusable type guards, validate API responses, and handle discriminated unions. Combine with assertion functions for invariant checks and generics for flexibility.