TypeScript Assertion Functions Guide

Assertion functions validate conditions at runtime and narrow types, throwing errors when assertions fail.

Basic Assertion Functions#

Type Assertion Functions#

Object Type Assertions#

Array Assertions#

Combining with Type Guards#

Class Instance Assertions#

Validation Chains#

Form Validation#

API Response Validation#

Environment Assertions#

Error Handling#

Best Practices#

Conclusion#

Assertion functions validate conditions at runtime and narrow TypeScript types in a single operation. Use them when validation failure should throw an error, unlike type guards which return boolean for conditional paths. Create specific assertion functions for your domain types, use descriptive error messages, and consider custom error types for better error handling. Combine with type guards when you need both validation patterns.