Type narrowing refines types within conditional blocks. Here's how to use it effectively.
Basic Type Guards
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Truthiness Narrowing
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Equality Narrowing
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in Operator Narrowing
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Custom Type Guards
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Discriminated Unions
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Assertion Functions
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Narrowing with Control Flow
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Advanced Patterns
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Best Practices
Type Guards:
✓ Use typeof for primitives
✓ Use instanceof for classes
✓ Use in for object properties
✓ Use discriminant for unions
Custom Guards:
✓ Return type predicate (is)
✓ Keep guards simple
✓ Throw for assertions
✓ Test edge cases
Control Flow:
✓ Use early returns
✓ Handle all union members
✓ Use never for exhaustiveness
✓ Be explicit about null
Avoid:
✗ Type assertions (as) instead of guards
✗ Complex nested conditions
✗ Ignoring null/undefined
✗ Assuming truthy means defined
Conclusion
TypeScript narrowing enables type-safe conditional logic. Use built-in type guards for common cases, custom type predicates for complex checks, and discriminated unions for state management. Always handle all union members and use exhaustiveness checking to catch missing cases.