TypeScript Narrowing Techniques

Type narrowing refines types within conditional blocks. Here's how to use it effectively.

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

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.

TypeScript Narrowing Techniques

Basic Type Guards#

Truthiness Narrowing#

Equality Narrowing#

in Operator Narrowing#

Custom Type Guards#

Discriminated Unions#

Assertion Functions#

Narrowing with Control Flow#

Advanced Patterns#

Best Practices#

Conclusion#

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