TypeScript Type Predicates Guide

Type predicates allow you to create custom type guards that narrow types at runtime. Here's how to use them.

Best Practices#

Design:
✓ Return boolean, narrow type
✓ Check all required properties
✓ Handle null and undefined
✓ Use descriptive names

Implementation:
✓ Validate all conditions
✓ Use in and typeof checks
✓ Consider instanceof for classes
✓ Combine guards for complex types

Patterns:
✓ API response validation
✓ Form data validation
✓ Filter array methods
✓ Discriminated unions

Avoid:
✗ Type casting inside guards
✗ Incomplete validation
✗ Overly complex predicates
✗ Side effects in guards

Type predicates create reusable type guards that narrow types at runtime. Use them for API validation, form data checking, discriminated unions, and filtering arrays. Keep predicates focused on type checking without side effects. Combine with generics for flexible, reusable guards across your codebase.

TypeScript Type Predicates Guide

Basic Type Predicate#

Object Type Guards#

Array Type Guards#

Discriminated Union Guards#

Null and Undefined Guards#

Class Instance Guards#

Combining Type Guards#

API Response Guards#

Form Data Guards#

JSON Type Guards#

Best Practices#

Conclusion#

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